So, ultimately here is what I did:
- Built a dome form (two half-circles of plywood, notched so they would interlock at a 90-degree angle)
- Cut an arc from plywood as a bending form
- Heated PVC and bent it around the bending form. Once I had a complete circle I placed it on the dome form. Did this four times for 4 different circle diameters.
- Laid chicken wire over the circles and wired it in place.
- Watered down yellow glue + brown construction paper to papier mache the outer form.
- With the second layer, I sprayed expanding foam onto the existing layer first and smeared it out before laying the second layer pieces down (i did this in sections - the foam sets up too quickly otherwise). Once this cured, it make a surprisingly rigid shell!
- Build a base and stood the hemisphere on end, on the base, and screwed it down.
- Single layer papier mache on the inner surface, also smearing the paper with spray foam before attaching it (so it would bond to the wire).