Pegasus has a 6" pry out deck plate over the clutch that a former owner installed.  I had to relace it
because it was cracked. The replacement has not leaked so far. You can now buy 8" deck plates at WM for about $15 which would be easier to work through. Bill Bell has suggested removing everything off the top of the engine (plugs, coil, etc.) and lay on the engine top to do stuffing box and clutch maintenance.  I've tried it - it also works.  If I didn't already have a hole to cover, I would not cut a new one. I now use the Bill Bell method.

My plate is as close to the stuffing box as someone could get it and, is as large as the sole will
take. Pegasus is a WC boat with a built in fuel tank under the cockpit sole so space is limited.
When I installed the replacement I wanted to use the 8" but was already at the forward sole bulkhead and the forward fuel tank bulkhead.  If I had the room I would install 2 6" plates over the stuffing box about 4-6 inches apart so I could get both arms in to make adjustments.

I cut a hole close to the size you indicate and used a Tempress 13" x 23" deck hatch for access.  This is a large hatch and pakes the access wonderful.  The hatch is plastic, but is pretty strong--much better than the similar Bomar inspection hatches.  This one is designed for foot traffic. The part number at West Marine is 490540.  Cost was about $82, so it's not cheap.  Seemed like a good overall compromise between size, strength, and cost.  If you really want to open the vault, there are some nice painted
aluminum hatches available--for around $300 - $400!  The smaller round deck plates will also work, but of course you have more limited space in which to work.

I installed a 1/2" balsa core in the cockpit to compat its inherent weakness, and even with the large
hatch installed it is very solid underfoot with no flexing.  Before I added the core, it creaked and groaned underfoot and was quite weak.  I don't think I would have wanted such a large hole in the original sole. I installed the core right over the top of the old floor (after grinding off all the gelcoat) and then glassed over the top of that.  Worked out very well

The forward edge of the hole on my boat is exactly 4" from the forward end of the cockpit.  This corresponds with the aft end of a transverse wooden stiffener that is glassed in beneath the cockpit (appears to be original). I don't know if that stiffener is present in all Tritons, or if its position
may be different.  In particular, I don't know whether this piece is present in West Coast Tritions.  It is worth worming you way in under the cockpit somehow to have a look before committing to any cuts.  The stiffener in my boat works well as it really helps stiffen the deck after the hole was cut.

Pictures at
http://www.triton381.com/cockpitstructure3.htm#COCKPIT%20SOLE%20ACCESS%20HAT

Bomar makes flush, rectangular cast aluminum hatches that are available with a white painted finish.  The cutout is 15 5/8" x 17 5/8".  The white one is available from Hamilton Marine--1-800-639-2715 (www.hamiltonmarine.com); part # BOM-C4T1416-W.  The price is $348.75.  They also offer the same hatch in unpainted aluminum (# BOM-C4T1416) for $318.75.  I considered these, and I'm sure the aluminumum construction would be beefy enough to keep your deck strong when installed.  They appear to be very nice looking, trim hatches--I have not seen one in person, only in the catalog.

Hamilton also has commercial Bomar hatches similar to the Baier hatches mentioned by Skip Baker.  These are very expensive, heavy and maybe unnecessary, although they would be the strongest part of your cockpit for sure.  You'd never worry about stepping on them--these are used in commercial fishing boats.  They're also very industrial looking, which may not bother you.  (BTW:  Hamilton is a commercial marine supplier and catalog retailer located here in Maine.  They have good prices and selection and
very good service.  They have a catalog you can order through the website or by calling; it's a good one to have in your collection.)

For what it's worth, the plastic Tempress hatch I installed is pretty sturdy underfoot-it flexes a little, but it is an acceptable amount and it does not feel like it would give way like the cheaper Bomar plastic hatches.  To me, it seemed like the best compromise between size, strength and cost.  Only time will tell if it is acceptable under normal sailing circumstances--I look forward to testing it beginning next season.  I may install a teak grate over the sole anyway.

The access plate on Ay Mon (original) is a round bronze plate with a screw cover. The diameter is about 8". It is by no means easy for me to work with (I am 6'2", 220 lbs), but I can get there. I believe the access was originally designed to allow you to work on your stuffing box as opposed to your tranny. Either way, you will still be working by the Braille method.

I would hesitate in cutting a large access hole in your cockpit. Even with stiffeners, I doubt that are would be as strong as one with a small access plate. You must remember that your cockpit floor takes a beating, In addition, it lacks the frame and/or bulkheads that give strength to the deck or the bridge deck. Installing stiffeners will also decrease the amount of space that you have beneath the cockpit floor. This is an area that I would preferred to have been larger (heck, I would have liked more space all around the engine).

As an alternative, why not access the tranny from inside of the port locker. Remove the plywood panel and lay in side of the locker where you can reach and even see some of the tranny. I have done it and while it is not my favorite place to be because of my size I can get it done. The worst part about it is removing all of my line and gear from the locker.

Another option (although more complicated) is to pull the engine. I have become quite good at doing so. When I want to work on the blasted thing, I rig up a 4:1 purchase (with my spare boom vang attached to some 2 x 6's across the top of the companionway) and pull the engine to the sole in the main salon (cabin). There, with a scrap of plywood to protect the sole, I can do whatever I want to the engine with the space to see and do all. Once you have pulled it a few times, you actually get good at it!