It honestly depends on how much you value your time.
You will see savings over the price of the Animal Plastics but not by a huge amount. ....I would say your most appropriate cage material would be plywood (PVC sheets are WAY too expensive to aquire unless you know someone). Plywood comes in 4x8 sheets so all of a sudden, a 30" cage width is going to cost you another sheet or two. Some people might tell you to use melamine but I highly disagree.....it is very heavy and ANY bit of moisture exposure to cut edges will destroy the cage. Large constrictors produce a lot of fluids.
Then, you're going to have to seal the cage bottom at the very minimum to prevent urine from destroying your cage. I like the idea of epoxy resin for that purpose.
Doors....glass or plexi (lexan).....both are somewhat pricey and difficult to cut. Then consider if you want sliding or hinged doors. If you do glass, you'll want sliding doors.
Then...venting. Ive used 3" plastic pvc drain inserts in the past for the purpose.
So....be prepared to spend a LOT of time on the build. Probably an entire weekend for most people if not more and a lot of trips to the hardware store. I didn't have any luck finding blueprints when I built my cages in the past, just look at pictures.
Personally, I like the idea of just ordering my cages now. Ive built a few in the past and just determined that extra hours of OT at work and ordering the cages would have been a significantly better financial choice lol.