How shallow are you looking? I consider my 40 gallon breeder to be shallow dimensions are 36" Wide x 18" Long x 16" High. And I’ve built all my tanks from scratch. The first things you really need are substrate because you want to add that before you fill the tank. And a filter to get it going. While it’s cycling you have plenty of time to think of what you want to design it with and go out and buy it
Agree with Quiche, love the Zoo Med Lowboy, but its price and apparent lack of availability have not made it possible to pick up. You can find custom tank builders online but they will cost a fair amount.
There are instructions online if you can find the glass or strip down some old tanks to make the dimensions you want, doesn't seem too hard overall to silicone the pieces together, the shallowness of the tank will make it easier as well as there wont be so much pressure on the seams. I have thought of taking two 20 longs before, pulling them apart and using the long sides to make a shallow square 30" long a side and 12" tall with a plywood bottom. Some day I will give this a shot.