I use to keep my hybrid outside during the winter here in montana, and it gets down to -40 F. I build a box around a cattle trough, and stuffed tons of insulation in it. Then I built an insulated lid for it also. Each half of the lid weighs around 75 lbs, and most of that is insulation. The walls are lined with styro insulation boards 2" thick, every crack in those and the wood is lined with great stuff foam, then there is double thick 18" insulation all around it. I kept the water at 82 F during one of the worst winters on record here. After the water was up to temp it barely ever turned on. I used one 500 watt titanium heater to keep it at that temp. Hope this helps some. If you are going to dig it into the ground you could bury a box, line that with styrofoam insulation then, the house batting, then a liner over top of that. You'd have to make an insulated lid or get a second hand jacuzzi cover.