I used the pink polystyrene foam 4' x 9' and it was about 1/2" thick. I thought this might not be thick enough but it sure is. I decided on foam as I felt I was not strong enough to haul sand, boy was I wrong. I also wanted the insulation factor for foam on the bottom. We have a clay soil under the foam that had 6" of round gravel so a bit hard to get all the gravel up. SOOOOOO One huge Gorrilla tape role and lots of foam and voila. (Pick a non windy day) I was so so careful taping and using 4 x 4 plywood to kneel on to tape so I wouldn't dent the foam. Then my goats paraded across leaving cute little goat hoof marks. Oh well. I was still careful. Then I used the cove foam, went well, used the wall foam- went well ( only sprayed adhesive at the top of the wall foam). Then some one said wall foam needs to go behind the cove foam-what ever it's backwards but it's fine. Wall foam hangs halfway on foam cove. It was SO SO hot installing. Then the liner, oh the liner-it was an overlap. It was super super hard to wrestle the liner into place on the foam. I swear the liner suctioned itself to the foam and was so so hard to pull, de wrinkle tug. The first time the liner looked great but as I filled ( the rain came and started with 3 in.) one side only had 1 " overlap. How? I can only figure when I was taking out wrinkles I did it all from one side.
SOOOOO I emptied the 12 " ( had to buy sump pump and did a cup and bucket at the end) and repositioned the liner AGAIN the dang liner was lopsided. I ended up with 3" on one side and 12" on the opposite. I said I give up and filled the pool hoping for the best. The liner held fine and I cut in the skimmer making any further adjustment impossible. I paid $119 for a 25 gauge liner from Poolsupplies.com no shipping either. In the end I was not very gentle with that liner, I was hoping it would tear but nope. SO if I killed the liner I was not to unhappy.
I did not use a gorilla pad nor a tarp. I have no idea why the liner was so so hard to maneuver over the polystyrene. It was very very hot, burned my feet when I tried wearing socks. OH I also gave up trying to not dent the foam. I put on sneakers and didn't care. Once I thought the liner was in place I added 4" water and went barefoot-a lot cooler but still hard to dewrinkle the liner There are no dents I can feel with the pool filled. The foam feels great. I don't think I would be happy with sand as tiny pebbles would worry me. Sand would have been a lot easier though. I did 98% of this install, alone.
So that's my long saga of wrestling. I lost 5 lbs doing it.
Now I hear the oval is harder. My pool liner seems were not where I would have expected. they were not equal and the seam was not help lining it up. Maybe expensive oval liners have better marks to help you get the liner in correctly.
Wishing luck and cool days, no wind too