The best place to start might be to get a scrap bike and modify the rear end, at least to make a prototype. You will need something steel and the cheaper the better from the point of view of it being easiest to weld to. Then I would just use 1.6mm wall square or round mild steel tubing to construct the back end and make it up as I went along. I guess you want stub axles so those will be interesting. They must be something you can just buy for other trikes.