Ok, I am trying to think out of the box here and what you are doing is not something I have tried. My thought is actually to build a box over around the stack of tubs. Bottom top and sides but the front. basically a rack body with out the shelves. I'd look at using a stiff foam insulation to do so. This is not likely to be enough alone but placing some heat source alone a long side should help raise the ambient temps (heat cable or CHEs two or three mounted inside but not too close to the tubs. The idea is to make a second micro climate. You have a one inside the tubs but then a second inside the room. It will allow you to heat the space the tubs are in and not be forced to try to heat the whole room.

Congratulations on the 2 legged addition!