I just finally finished my big winter DIY project of making a "dry" box out of wood. I put dry in quotes but due to the gasket materials I used I actually feel pretty confident on the waterproofness. The inner gasket is a waterproof closed cell foam that regains it's shape with no pressure on it and conforms to the top lip when compressed.
It's 1/2" Baltic birch plywood for the sides, 3/4" on the top and bottom surface. Polyurethane coated inside and "deckote" outside (which I have on other rafting gear and has proven very robust). Rope handles with pvc tubing. Stainless 24" piano hinge and stainless butterfly latches from Amazon.
I am not a carpenter by any stretch and mistakes were made but I went slow and apparently I have the skills to correct those mistakes to get to a "good enough" level of qc.
Reason for building is I'm cheap and shipping to Canada is insane for something that size, and also I like being able to make the exact right size for my setup, which is not always easy with aluminum dry boxes.
This will be my seat with the lime green box pad from stitchesnstuff.