I actually built this one-person plywood hot tub featured in Mother Earth News, once upon a time. It will fit into some rental bathtubs and showers, but not others. I put it up on two long blocks, to distribute the weight.
The tub is surprisingly comfortable, because of your own buoyancy, and the water stays plenty hot through a good, long bath. Contrast that with the so-called bathtub in our current rental, where you can lie down on the bottom when it is full and still breathe, and the water temperature drops rapidly. (Nine inches of water in the deep end.)
This hot tub is also simple to build, even with only hand tools. I had the lumberyard cut the plywood. I used polyurethane to seal and waterproof it, and I should have used wood filler at the joints, as the polyurethane ran through and didn't fill them very easily.
When I couldn't use it as a hot tub, I used it as a base for my desk top.
I gave it up in the Big Purge, but I plan to build another, later on when I have time to take long baths again.