I was at my LHS yesterday and saw something that could answer your problem, if you really want to go with white step plates. Evergreen company makes all sorts of styrene shapes and one I saw was "metal roofing". I is .040 thick, a little over 1/32", and comes in about six different sizes of 'corrugation' groove sizes/spaces. At least one of those groove space should work for your boat and building a mahogany frame for the step pad should be no problem. I hope your LHS sells Evergreen Scale Model products.

Bob