What would one use 300 gallons of rainwater for? I’m not trying to sound judgemental or whatever, I’m actually curious because 300 sounds like a lot. Is it mainly for irrigation? Would that make sense for a larger garden? I’ve heard that some people use graywater for flushing toilets, is this part of that?

Have you asked a brick-and-mortar eyeglass shop how much they’d do it for? Independently run places tend to be a lot more accommodating in my experience, and asking’s free. Considering how much of a PITA cutting and drilling your own lenses sounds like it would be (not to mention the possibility of breaking them in the process and having to get new ones), it’s worth asking a couple of different shops first.