I think it's probably time to start building and testing your ideas!
It sounds like you're aiming for a step-through (lady's?) bike with a development of what you've already been trialling in the coroplast.
I'd be using a blown acrylic dome and lightweight frame (laminated plywood strips?) with stretchy cloth and zippers, on one or other of these:
(My restored/modified 1960s Moulton "billiard cue")
or
(My work-in-progress Moulton space frame.)
But neither is common or particularly attractive to many people.
Good luck - I think it is a great project. (I never realised how hard it is to improve the efficiency of a machine until I started working on pedal-powered vehicles... makes car stuff look easy.)