Welcome to the forum. If you are not going to put any sort of bracing(plywood) under the foam, I would double up the thickness. This will give you more rigidity and also allow you to carve down into the foam to create below track contours. To find out everything you need to know on this and other subjects, try the search function. There are countless threads on this very subject. I believe there are a few current ones in the 'Layout' section as we speak. Good luck!