Don----in regards to your "potty holes"----I have built seats using this technique, and it really helps with the comfort factor in a car with home-made seats. My first 2 rods that I built were either chopped, channeled, or both. In a coupe, to avoid the "bent neck syndrome" and in a roadster to avoid the "head sticking above windshield" syndrome, I has to make the seat bases extremely low, using high density foam and 3/4" plywood, which sat right flat on the floor. If you just use foam and plywood, after the first 10 miles of riding the high density foam "solids out", that is to say there is no farther compressibility in it. Every road shock is then transmitted directly through the plywood to the end of your spine, and it gets uncomfortable darn fast. To use your so aptly named "potty hole method", an area roughly the size of your fundament is cut out of the plywood frame, and some woven, flat, non-stretch material is "woven" across the hole in 2 directions, then heavily glued and stapled to the plywood perimeter. That way, the compressed high density foam gives a uniformly distributed load on the woven material, the woven material "soaks up" the road shock (your butt is for all intents and purposes suspended in a "hanging basket", and it saves the road shock on the end of your spine. It really works very well.
Brian Rupnow