Seeking recommendations: Robust rear triangle for a DIY suspension cargo trike?

Hi everyone,

I’m currently in the middle of designing and building a custom cargo tricycle. My priority is comfort; I’m including a suspension system specifically to ensure my old dog has a smooth ride.

While the front end is coming together, I’m stuck on the rear triangle. I’m looking for a robust, "off-the-shelf" solution rather than hacking apart a donor MTB frame, as I’d like the build to be as repeatable and clean as possible.

Current Considerations:

• I looked at the Kinetics MTBrompton rear triangle, but at £445, it’s quite pricey.

• I’m also concerned about whether a folding-style rear triangle can handle the weight and lateral stresses of a cargo trike.

My Criteria:

1. Durability: Needs to support the weight.

2. Suspension Compatible.

3. Availability: Ideally an "off-the-shelf" part or a kit that can be purchased independently.

Does anyone know of any manufacturers or specific aftermarket rear triangles (MTB or otherwise) that would be suitable for a heavy-duty cargo build?

Thanks in advance for the help!