Mark, nice looking plan, I think Lew is on the right path. If you use a support a 1/4" taller would give you a 1/2" clearance then you could use anything you wanted for road bed. Even if you just used a thin steel plate as it is the bottom clearance would be the same as you have now.
If you go with both a 1/4" along with a thin product you will be golden! If you need I have some used I think its 1/8th or 1/4" plexi glass I could cut your curve into with no problem.
Either way its looking good!