18 pounds! ^o^! You are doing an excellent job with him.

Properly made PVC pipe-framed habs are fine. PVC needs paint to protect it from UVB, or it degrades fast. With age, PVC becomes more brittle than it was when new.

Can you afford to make one out of 1/4 inch Plexiglas(clear acrylic) sides?

If so, I would use the strongest PVC frame material commonly available- 2 inch sched. 40 PVC pipe. You may be able to locate those 90 degree PVC corners that have a socket at 90 degrees to the other two, which would be the main 8 corners. Glue the joints properly and they connections will be plenty strong.

This is the only way to do it: Prep the pipe ends, bends & couplings by thoroughly sanding the inside of the sockets and the outside of the pipe ends with 180 grit sandpaper, prime the end and the socket with blue primer, then paint on the clear PVC glue. Insert the pipes firmly with a 45 degree twist. Hold the pipe in the socket for 30 seconds. Do each glue-up as quickly as possible so the glue doesn't begin to dry while you are painting it on.

It's too many words to explain how to make a reasonably airproof door for cold weather, so only if you want me to, I'll gladly post it here.

Regular drill bits crack acrylic, so use a special drill bit to pre-drill the screw-holes in the acrylic, and size the bit 1/16th larger diameter than the diameter of the screw-shank(so when the sheets expand/contract, they are less likely to crack). Don't tighten the screws, so they don't restrain the acrylic sheets. Leave 1/16th clearance between the heads and the sheet. This doesn't make the sheets feel loose. You can get the special drill bits from Tap Plastics or tapplastics.com. I would use 1" #10 pan-head sheet metal screws, and not the self-drilling(self-tapping) kind.

If you can supply a pic, I can advise you on ways to rehabilitate your current iguana shelter.