Hi tommygirl
I'am a carpenter and build high dollare homes.if you are trying too keep the temps in the res at one temp I would insulate the bottom of my tub with two pices of foam board.they are 3/4" thick and come in 4'x8' sheets.They have a silver foil on both sides.very easy to cut.You can cut the board and make a box that fits the res,glue it together with a tube of silcone and a couple of nails.than set your res down in it.Or you can get one or two cans of foam insulation in a can.WEAR RUBBER GLOVES when you spray this stuff or you will wear it off your hands.Turn the res upside down and spary the foam on the tub.Bottom abd sides.The foam will expand two or three times it size.Let it cure and you can shape it any way you like.It is so lite in weight you will only add two or three lbs. to the tub.After it's cure shaped and ready you can paint it with a water base paint,water base only.
Hope this helps and not to confuseing. The foam board and spary foam you can pick up at most home building stores,hardware store{homedepot & lowes} .