I'm more or less in El-Ziggy's camp, though I'd argue that you should budget even more space for your snakes. [I have very high standards for reptile housing, so keep that in mind and make of the following what you will.]

Going off AP sizes, I'd personally consider a T100 a minimum for an Indigo or a Cribo -- they're big, active Colubrids, they'll use all that space. You could probably get away with using T70s for them if you're pressed for space and need to stack. For the scrub and the (presumably mainland) retic, you'd ideally have small rooms (think 12' x something) over PVC cages. Ultimately, they're both be active 12+ foot giants, so even a T100 would be too small (by my standards) by the time they're done growing. I'd be happy to more of "WrongPython's Thoughts on Housing Giants," but I'll leave things there for now for brevity's sake.

If I were in your shoes, I'd prioritize your hunt for a new place where you'll have a larger snake room(s). That closet might not be sufficient space for much longer given how fast retics can grow. I'd get one or two of the 8' enclosures as soon as possible and divide them up between your snakes. As El-Ziggy said, given how much the PVC enclosures cost, this will be the most cost-efficient route in the long term. I think you'd ultimately be happier spending more money on good PVC enclosures over building plywood ones -- they'll last longer, look a bit nicer/more uniform, and retain heat and humidity better. That said, PVC enclosures of the dimensions I mentioned are wicked expensive, and properly constructed plywood enclosures are by no means bad enclosures. If giving your snakes the space they needs means going the plywood route, then go for it!