Just wanted to chime in to say that we put 8" x 16" x 1.75" patio/paver bricks under the legs of our 20' x 52" intex ultra-frame pool and it made a *huge* difference... Everything stayed level to within *1cm.* this go-round (yay! :} )where in previous years we have struggled to keep the pool level to within a couple of *inches*

We also used 1/2" pink extruded-foam insulation sheets taped together as a base under the pool. In the past we have used 1" styrofoam sheets under the pool and it also worked ok. Main difference was that the regular styrofoam seemed to be more fragile during assembly and developed a slight mildewy odor that was noticeable once the pool was taken down... My guess is that it was slightly absorbent and soaked up some H2O. The pink foam is denser and doesn't absorb H2O at all... but it's around 30-40% more expensive so go with what fits your budget, because either will work.
(I'm hoping to leave this pool up year-round this time since before we had an easy-set and this year upgraded to the ultra-frame, so I thought the denser foam was a better idea... Time will tell)
One other tip that I would offer is that the foam really needs to be a little larger than the bottom of the pool, but (if you are using rigid foam insulation) should fit just barely inside the legs of the pool. This prevents the bottom edge of the pool from rubbing on the edge of the brick. We buried the pavers in the ground but left 1/2" above grade to account for the thickness of the foam. If your foam is thinner, you could probably just bury them flush with the ground, but I'd still make sure the foam extended over the edge of the pavers.
Good luck!