This is a project I want to take on this winter or early spring so will look forward to see pictures.
I just ended up buying a Kodiak livewell to get me through the rest of the season. I'd think about going fancy, have an inverted U channel type lip to grab the water so it doesn't slosh around and in the corners maybe try to put in a way to circulate water rather than just dumping it in.
The worst part about some custom livewell's is the sloshing around of water on deck, case and example why I just bought a small one this year. I do not want to be out in Cape Cod Bay having water all over my deck from my 55 gallon drum cut down. Not only do my feet get wet (do not like always wearing boots as I think they can be a safety issue) but also its landing all over the deck hatches which are secure but its just not good seamanship. Never mind if your feet get wet now, its freaking cold water.
In my past boat H&H 40 we had a pretty good setup but wasn't dry. It was a carolina style box in the back, one I'd like to repeat. Corners were not rounded on the inside but this thing was huge, I think we could fit 400lbs of ice in it, might have had some insulation. It had some nice touches, it had thru-hulls going through the top and draining water naturally outside. It also had a small lip aroudn the bottom, almost like a cut out for your toes, so you could get really close to the back of the transom, toes under this rail also gave you stability while stand up fishing. In the corner of this was a typical drain plug found on a skiff to drain the water. I wish I had good pictures of this.
I wonder if you would want to make it out of some foam material instead of plywood, was just looking at one forget the trade name but also would give it insulation, so if it wasn't used for a livewell can double as ice storage, etc.
I've been thinking about ordering some of stuff from Jamestown Distributors to get practicing for some projects...