Acrylic bath installation

I was going to ask a similar question so i will add to this instead.

I have been looking at the expanding foam for my install. I was looking to use it just around the walls of the bath/spa.

From what i have read putting it under the bath can be a problem. If you are not careful it can lift up the tub. One person i was reading had a lump in the middle of the bath from a pro using this. Others recommended against using it under the base as it will fall apart over time, leaving a depression.

I am trying to work out how i can better insulate the floor of my spa. If i put it in a mortar bed on a concrete slab i would think i will lose a heap of heat out the base.

Maybe a sheet of wood with a morter bed between it and the tub? Could you glue a rubber sheet under the tub before setting it on a mortar bed?