I used "spray on" contact cement,
and regular R-13 pink fiberglass attic insulation.
After I sprayed the fiberglass,
and the outside of the tub,
I applied the R-13
and then sprayed the outside again,
and wrapped it in space blankets
I bought on ebay.
Foamboard insulation will be more energy efficient,
but may cost more,
I haven't priced it that way.
Also, you could buy two tubs,
and when they are stacked,
fill spray in foam
in between the stacked tubs.
This method would make an "Igloo Cooler" type rez.
Perfecting this insulating method
might take some trial and error.
.