Are you looking for companies in general? or specifically members of this site? Synergy reef, elite aquatics, lifereef are just a tiny handfull of custom sump makers. This also greatly depends on what you want your sump made of, glass or acrylic being the two main materials. Typically when it comes to custom each manufacturer will only deal with one type of material, The most common being acrylic due to its flexibility.
buy a glass tank of your choice. Get with a local glass company (every town has one usually) and have them cut you the inserts for the dimensions needed. Silicone them in. They could probably do it for you. Most don't realize how quick and easy this is, but more importantly its cheap, and gives you the opportunity to lay it out how you see fit.