Thanks! Got a few quotes from the places mentioned - custom is expensive
Although I'm sure everyone here already knew that! For tanks, it's about 2.5x the cost of a rectangular one, and that's just for a plain rectangle with a single corner angled off.
So I gave up on squeezing every gallon out of the available space and went with a rectangular tank from a local shop, Marine Sanitation in Seattle. They were helpful in figuring out all of the fittings, and it was super valuable to see everything in person. Also nice to have a local person to talk to if/when something goes wrong! (Knock on wood)
Thanks for all the help,