Welp.

I did a lot of calculations and diagramming today. A lot. My wrist even hurts.
I had this awesome, awesome idea for two ponds, side-by-side in an L shape, one rectangle and one square, like aquariums that fit the layout of the stuff in my garage. One 6x4x3, one 3x3x3. They would share a basking area but would still be separately enclosed. One would be 500+ gallons, one would be 200ish gallons. I even had the turtles' gallons-per-turtle needs calculated and a list of turtles that would fit into each pond. And I was on my way to building a waterproof container for my webcam.
I did all the layout, the structural designing, the price-checking / list-making on Home Depot's website, and all the calculations for the pond liner and found someone on Craigslist that was selling the pond liner thickness I needed.
Total price for a DIY set of tanks for all turtles in my pond, totaling about 700 gallons: A bit over $500 and a whole lot of weekend work and likely some injury.

Now, there was someone on Craigslist that I found that was selling his 4x5x2.5 300ish gallon above-ground koi pond with liner and pump and filter for $350. And I found a place online who would sell me a 300 gallon Rubbermaid stock tank for $250 with free shipping.
Total price for a buy-it-now-no-assembly-required-don't-even-touch-a-hammer-immediate-gratification 600-ish-gallon setup: $600
... so it looks like the DIY Super Garage Aquarium of Doom Project is on hold for now, but my turtles will be safe and healthy inside for about the same amount of money. Heck, even less money when you factor in the extra cost for broken parts, redoing of things, bandaids, new tools that I would need, and a whole assortment of other things. AND there's no chance of a sudden, explosive leak / drain ruining everything on the floor in my garage, either.