We have a 12 x 24 Ultra Frame. We poured a five inch concrete slab with 1/2" rebar placed in a square pattern, and set the pool on it. A competent concrete worker could verify this for you. You are correct, it is much easier to setup and take down. Leveling was a breeze and there is no changes in level from settling over time as when we had set the pool on compacted soil.. A side perk is that less dirt gets tracked in as well. I've attached a diagram from Intex that lays out the footprint of the Ultra Frame, including the legs. You will need something between the liner and the concrete. We didn't use foam boards, (the proper type of foam boards were not available in SoCal and two inch foam seemed like too much thickness to me). So instead, we used 1/2" thick 24" x 24" Fatigue Mats that connect together. They came at a reasonable price from Harbor Freight, but they can be found all over the place. We also taped all the joints to insure the mats didn't shift or separate while filling or through use over time. They also do not absorb water, which I feel is a plus. The 1/2" thick fatigue mats were more than enough to take the hardness and rough feel from underfoot and to protect the liner from any unnecessary abrasion. Between the fatigue mats and the liner, I used a Gorilla Pad as well.
The mats really hold there shape. Mine didn't go flat at all...In addition, Before I laid down the Fatigue Mats I filled the concrete expansion joints with a flexible caulk to avoid any irregularities on the bottom of the pool. Hope this gets you going..
