PVC tubing is extemely flexible in the cold water grades.
The better grades - including electrical conduit is also too flexible to use in aircraft.
Instead of re-inventing the wheel, why not copy an existing Rogallo hang glider?
Here's one look at a Rogallo: en.wikipedia.org...
Once you get the aluminum tubing you need, you could order splice T's and the like from the hang glider kit companies.
Assembling things with JB Weld - an excellent metal epoxy adhesive - would work well.
There are other alloy specific adhesives out there, but JB works well in most areas.
I've combined aluminum fuel line fittings with JB so as to make a fitting that isn't available.
The interesting part of this engineering exercise will be setting it up so the bike is steerable when running in ground mode and the wing's control
bar is operable when in flight.
Combining the two controls in some way would help to alleviate the weight problem.
Using a very lightweight bike to start with would be a given.
You should be able to find a lightweight 10 speed road bike out there, used, not too expensive and use that in lieu of a heavier mountain bike.
You've probably thought it out, but using a subtle and seldom used by auto's modest downgrade would be a good way to go for the "taxi" tests and
the very low level liftoff testing that would be required prior to flight at any altitude over 3'-5' or so.
One thing you do want to incorporate is an adjustable hanger system that would allow fore and aft CG changes with ease.
A simple strap with multiple holes drilled within and welded to the framework along with a "pit pin" as used in race cars or a bolt of proper
strength with nylock nut would make CG changes easy.
Use care here, I read about one home-made hang glider where the builder used cheap dime store bolts to support his weight and when the bolt broke, the
CG change was tremendous and he rode a fluttering piece of nylon to his death.
Follow the lead of aircraft manufacturers.
Use adequate strength materials and do not settle for sloppy workmanship.
Uttering "Uh Oh" at 3000' is no fun....