Well unless you have the space and tools, it might be better to just buy one. And for an adult boa, i recommend a T25. That is what i use for my adult boas. The babies and juveniles get T8 and T10.
Plus if you make a mistake, it is on your head to eat the replacement cost of materials. If someone else makes the mistake, doesnt cost you any extra lol.
I personally think AP prices are very reasonable considering the time it would take me to make one without a workbench and routers. The killer is the shipping on the big ones.
As for saving cash, the bigger you go, the more you would save doing it yourself. I mean at $150 for a T8, the cost of materials would probably easily be at least $100. 1 sheet of 1/2 inch CDPVC and 2 panes of glass. Bet that isnt that cheap.