I think you've got a pretty good idea there.

I'll leave the fiberglass questions to others, but as far as the
stability when towing, You might consider adding a skeg in the last
quarter of the "trailer". Should keep things all lined up and travelling
in the same direction as the tow vehicle.

Also, remember that your spray will be hitting this thing square in the
face. Plenty of gaskets on the door would be a good idea.

Chuck