IMO (u prolly not lke this) you should custom build your own or order one.
I have built acrylic sumps before to maxime every inch of space under a display tank. If you are placing enough baffles & bracing you can use cell cast acrylic one size less than the suggested thickness for an acrylic aquarium hieght. You can also build one out of glass yourslef. but this all takes time and $$$.
so maybe if you dont what to go through a custom then my other suggestion is going on criagslist/ebay to see if anyone nearby is selling a 40Long 48"x12x16
per aqueon products info thier 40 Long is 48ľ x 12ĺ x 167⁄8"