Just remember that flexwatt cant be trapped without ventilation. The manufacturer suggest a 1/4" of room to let any hot spots dissipate to reduce fire hazards.

When we had a single tub in use, we got styrene (styron? not sure lol) foam board you can purchase from home depot, and made a little box that could go over the tub to help with heat distribution.

But, I will be the first to tell you that if you have to, putting a strip of heat tape down the length of the tub to help raise ambient is fine. Just make sure that it never gets too warm. You're only looking to raise it a few degrees.