We designed a system that used no rails and the adjustment plate for the tension was just 2 screws. It is built into the frame of the unit though not something you can retrofit.
If you could design a plate that could handle most larger engines like 18hp on up, several brands (template for drilling the holes on the plate) and your tensioner on the plate, then maybe contractors would be interested?
It is easy to drill holes in a plate, the hard part is drilling holes for both the engine and pump/rails so that they are straight and aligned so you can bolt the pump and engine on, attach belts and tighten.
It would be nice if you could order a plate that is ready to use or almost ready to use to mount your engine and pump and attach belts, tighten and you are ready to work.
I am sure that there are bolt pattern templates out there for the different size engines and probably for the rail systems also.
Even if you just marked the holes where to drill so that you could attach the components, it would be a huge help for the many that build their own rigs.
Is your pump base something that could be a separate thing Paul that you could sell without the frame?
Lots of guys would like to build their own rigs but the hardest part is drilling the holes so that everything matches up and no belts pop off when running.
Maybe sell templates out of aluminum or plastic for the different engine brands/sizes so that the holes are easy to drill?
Maybe sell the plate with the pump tensioner and bolt pattern plates for the engine the builder wants to use?
Just some ideas.