Here is something that just came to me... somewhat using several of the ideas together...

Mount some bed rail fasteners on the FRP walls behind the filler face, so there is a rail that goes across the gap, behind the face. Then put a wide foldable step on a wood piece that goes from fastener to fastener.

This means the whole arrangement could be removed, or just hidden behind the face door, when the step is folded up. I think this would also allow for a wider step which would be more comfortable to use and the step could extend out beyond the face when folded down for use.

I'll have to mock this up with cardboard and give it a shot.