You may have seen mention of a mortar base that the tub can be dropped into.
It doesn't take much, just some piles that will smoosh down.
If you try to set it down uniform, the tub may not push down enough to meet the deck.
If you have a "deck" tub, it's a good idea to support the bottom, the lip isn't intended to carry the weight.
Insulation around the box may help to keep heat in.