Hi, BJ, I hope you don't mind an idea. I have looked on The Google for "DIY case kits" and not found anything. If someone sold all the hardware and a simple instruction sheet for putting it all together, people could potentially save money by purchasing a kit, buying their lumber locally, and doing their own assembly. Obviously most people will not own a rivet gun, but if you sold a kit and plan that customers could put together with common power tools, that could be a good little business.
It costs over $100 to UPS a mid-sized rack case or road trunk. Just reducing that to $20 for shipping a box of casters, ball corners, rails, screws, etc. might give people with spare time a good cost savings. But it's a pain to find all that hardware. Since you have already done most of the work ... draw some plans and sell kits.