I had a tank back in the 80's that was actually pretty simple. The outside was actually a big cardboard box about 5'x4'x10'. The bottom 18" up to about the waterline was doubled for extra strength and then there was a frame of 2x4's that went around the outside at the 18" mark (top of the doubled section) to hold against the outward pressure of the water. The front was cut back at about a 60 degree angle and the access door was there. Inside the whole box was lined with about 8 inches of styrofoam for both sound and heat insulation. Then the whole thing was lined with the same type of plastic used for water beds. A standard waterbed heater was installed under the bottom plastic lining. The door was a rigid plastic with the same 8" of foam inside it. The other cost in the tank was a pump and filter. As this was a low-cost tank the filter & pump were external and had to be placed in the tank after every so many hours of use to filter out mostly hair and dead skin cells. The downside of this is that the door was the only access and was slightly ajar during the filtering causing the tank to lose heat. I would try to build this into the system If I build a tank some day.One of the main tricks in getting an undisturbed float was eliminating drips from condensation on the ceiling of the tank. You can wipe down the ceiling before a float to mitigate that. However, if you are building a tank from scratch it might be worth trying to arch the top and really smooth the plastic on the ceiling so that condensation drops will slide down the ceiling and sides instead of dripping. That would be more work, and as I haven't done it I can't say it would work for sure. Just a thought.