I have been thinking about the same thing lately. Here are my thoughts.
1) fiberglass-- especially a drill hole is easily filled in and sanded over,
if your any good no one needs to know.
2) there is also a type of rack that mounts to the top. they are made for
camper shells. real cool-- also has a shovel/ax mount.
3) mount steel backing plates on the inside for a metal to metal sandwich.

Todd
84 Westy
washington DC