Elevated foundation on base board

I'm starting with a 15/32" thick plywood base board measuring roughly 24" square purchased at Home Depot. Once I get through the first few rounds of dry fitting, I can determine if I will cut the base board down from this original size.

Even though I will be working with a 1:12 scale model car, I'm still going to cut the kit down a little to better fit my idea. I'd also like to have an elevated foundation, so I won't be using the existing foundation pieces from the kit.

I tend to keep unused and/or extra pieces from previous kits, so I dug out the foundation pieces from the Foxhall Conservatory -- the kit I used to make the Newport garage. These are 1 3/4" tall and form a rectangle that measures 13 5/8" x 14 3/8". This foundation is just a bit too short depth wise, but that's easy to remedy.