Hi
Curious as to why you want a "custom" Unusual size o r shape.?? Probably the lowest price would be to make it yourself from Acrylic particularly for a tank over 150 gallons. Smaller than than it wold probably cost you more.
Now if you want Bowfront Round Fisheye Cylinder, there are manufactures of these but expect to pay at least 5 times the volume price.
Must be dozens of custom makers in your area. I would certainly get a stand and filtration system in the quote and installation particularly if very large or placed in an unusual location. As you've probably already noted shipping will cost a fortune. Would guess it will still cost at least 3 times a standard tamnk??? Good luck
Would like to hear more about it as you progress ??? gary

A custom cube aquarium (that's what I'm looking to create), without rims, is very modern and unusual. I already have one, but paid too much as I wanted it quickly. I just don't like acrylic, looks a bit inexpensive, scratches, etc.

Yeah, if I have to, I'll pay for shipping, but hopefully I can find someone around here. You would think it would be easy to find a good price in a huge city right?

Actually, I did get a very reasonable quote of $170 for the 18" cube aquarium (with glass top) I am seeking. I don't think anyone online can beat that as shipping will cost a small fortune.

Try Pacific Aquarium by China town

THANKS! I've gotten a few reasonable estimates, but Chinatown screams deal!!

Also if you want to try Thousand Island Aquarium in Brooklyn. They were making some Unusual shapes they will probally deliver and set up.