Peterson's will build anything you ask them to, but they won't take on the liability. So if you tell them to make a 300g tank with 1/4" acrylic, that's exactly what they will make for you... and it will bow out and split its seams the moment you fill it with water (and it will not be their fault). If you're looking for small tanks or sumps and you know how to design them, you could have them made at Peterson's. Otherwise you should probably go to someone who specializes in aquariums. (And I'm actually kind of interested in this question too, so I'll be tagging along.)