Chip,
The layout looks vey similar to my last seat.

I drill a few holes in the bottom of the seat for two things. One to let water drain through so you aren't sitting in a puddle and second to gauge how close I am from sanding right through the foam. I try to get it to at least 1/4" thick so center of gravity is as low as possible. I also don't permanently attach the pieces to the boat so I make them oversize and wedge everything in. Although the foam is dense it does compress over time, so make the pieces a little thicker so they don't become loose after several hours of paddling. It will all bounce back into shape over time when you take the pieces out to clean your cockpit and store boat.