The key to good foley pits is isolation and quality of sound. Good foley pits are expensive to make but the investment is worth it. The kind of wood you use to box the pit is crucial to the acoustics of the pit as well as durability. Hardwood like oak, teak, seringa are good. Your comment about using rubber insulation is spot on. But you can use the rubber insulation between your traps, shielding them from each other so they don't bleed into each other especially if you are doing foley with another person and using 2 different surfaces. Pit depth is also crucial. Don't make them too deep- around 350mm- and with the added surface such as hard packed gravel or pebble on had packed gravel, your depth should be around 50mm from the top.
As far as the insulation of the bottom is concerned, foam rubber padding or soft rubber (15mm thick) is good. Cement is a really difficult surface to design. If it is too thin, it will ring and sound too thin. If you have the space, make a large cement surface and then have an area that has a varnish on it and an area that mimics exterior pavement. Also, have a small depression in the exterior one when creating puddles. I have seen studio's use hard rubber as their side insulation and the depth of the cement is quite deep (300-350mm).
Foam rubber (the orange stuff I think) is also a good insulator.
Hope it helps.
