There are 2 school of thought....
One of them is the "drop box" that allows the boat to essentially float ON the water and you to be below the surface in the box..
The down side, they don't tow well.
The other is kind of like an upside down contact lens floating on the surface of the water. This will have the rim of the boat floating above the surface when you are out of it, but then go down and disappear once you get into the boat.
It is a few degrees away from FLAT and may even appear flat on land, but all that displacement will have it float right.
I would cut the softest arch out of a 16' piece of decking and use it as a strong back and make your bottom a big oval from there, then rib it. remove the strong back and then deck it with as high a dome as you think you'll need (remember to measure your buddies big feet when making the stern end)
You will get a million suggestions, all unique to the place hunted and the guys hunting.
But if you want a low tech, backyard way to do it, that's it....
"I've been left for dead before but I'll still fight on, don't wait up, leave the light on, I'll be home soon"