I have started construction on the Aperture Science Hand-held Portal Device. The 4-inch wide telescoping mailing tube came, and it turns out this will be an even bigger boon than I originally anticipated. The telescoping mailing tube, which I thought only telescoped at the front, completely separated after I cut the end off. This means I can use the outer shell as the proximal holding piece and don't need to buy larger stuff. It is possible that I will change my mind after seeing how the different thicknesses compare - worst case, I buy two couplers of PVC pipe which are already the proper length and require no cutting.
On another note, at this point I'm glad I went with spackle paste instead of Bondo - a couple reasons. First, it takes longer to dry. This means I have longer to work with it and if I make mistakes I can easily fix it. Second, since I'm not using PVC pipes for the more complicated parts, spackle should be plenty sufficient, where Bondo would have been overkill. Third, it washes off my hands super easily. Tomorrow I'll add on the wedges, after it's dry and I can superglue the base of the molding cards to part of the device. I coated the inside of the device also with spackle, so I could sand it down and give it a better surface for spray painting. We'll see if it sticks or just flakes off.
Ended up cutting most of it with a combination of the exacto knife and a handsaw. Worked, but not super great - will need to clean up the edges a fair amount, but that shouldn't be too hard once the thing is mostly plaster. When this part is complete, I'll get either use the telescoping part or buy some PVC pipes to get part two of the main frame figured out. The third frame portion is the most challenging... it's the nozzle so-to-speak - but since the spackle is so easy to work with, this may work out okay. The only concern I have at this moment is that there may be too much moisture in the spackle and it could deform the thick cardboard, but I have it in lid of the telescoping tube to hopefully keep its shape overnight.