I had to dig into this topic when I cracked my Black Water Tank stub clean off as it bottomed out on a Road dip. Ah, the drawbacks of rear TTT Plumbing, and a low slung TTT, soon to get an Axle flip [or Shackles]...
The CO Repairman who helped me out fixes many similar problems. He notes that standard procedure is to get a Tank, linked below, and drill out the holes req'd. One for the Toilet stub, and one for the Vent. On ABS Tanks, as I have, you can use ABS Glue, as used on ABS House Plumbing. In repair situations, he said they like flexible couplings, as used when repairing Waste Pipe sections back together. There are 2 large Hose Clamp, screw-tight fittings, as on Radiator Hoses.
As posted here, I believe, the standard main fitting is identical to that on a House Toilet. I've used a 'newer' Wax Seal to seal a House Toilet to the Flange. It has an integral, curved piece to ensure flow down into your ABS Flange and the Tank.
A custom, leak-proof Tank might be tricky to find. FYI, my Tank was repaired by 'ABS Welding'. Hot Gas is put to the Tank, and new Material was fed off a Spool onto the heated area where it becomes 'one'; good as new. I'd never heard of such a process, so I looked it up on Wikipedia. Perhaps such a method might have to be used to modify one side of a standard Tank so that will fit in the space you have. This Repairman told me that fabricating up such a Tank from scratch is very standard procedure, and is done to replace older Tanks of 'odd' dimensions.
As is the case on Wood-framed House Floors, you'd want to really make solid the area around the main Toilet Flange so that it doesn't flex with Human weight. Given my own 'unique' experience, I'd also want to use secure Fittings that can be un-coupled for repair/replacement.
A clean Water test run would be well advised once your install is complete!
Black Water Tank Supplier
Black Water Tank Dimensions
