[jvriezen]

I must say, however, you have very nice looking bumpers.

A fastening technique we have used in the past is to use T-nuts (available at your local hardware store) installed on the inside face of the plywood (you'd have to disassemble for this) and then hex head bolts that go through the frame into the T-nuts (short enough to not go poke through the plywood!). Very tight connection, can be removed quickly with a socket wrench or if in a tight spot just a box end wrench)

https://www.google.com/search?q=t-Nut