Generically speaking I have tossed many/most of those ideas around.

Problem is, I do not want to have to load from the rear. Truck, trailer, and a kayak being loaded are going to need 50+ feet of space. Someone parks behind you on the side of road, you can't load or unload. So I pretty much eliminated that idea. That means side doors and they need to be 18 foot long. That's where the problems start.

The other big issue is a rigid framework that is self supporting and not going to twist and rack as it goes down the road. Metal makes a lot of sense here, but there goes the cheap part and there would be a lot of work in building that.

I thought about PVC pipe but that's not cheap either. There is still the structural issue over the doors. That needs to be truss and/or sheathed in plywood.

Can it be done? SURE! I am just not sure it's worth it to me. I can build a rack and modify by trailer for probably $300 or less. This is a lot more work and money. Not giving up, but I don't see this fitting into my budget.