Thanks, EMD.
I think those hand grip heaters will work very well for spot heating. Apparently they come in both 3w and 10w. I'm like you though; I prefer slow continuous heat over full-on, full-off heat.
And yes, boxing in the tanks will make a huge difference. I would use a combination of closed cell (blue)styrofoam and foil backed bubble wrap. (Or even a layer of your basic lightweight tinfoil would make a big difference in heat loss. Just remember, the thinner the better, when it comes to reflecting heat). However, the bubble wrap stuff is equivalent to about 1 1/2" of rigid, IIRC.
If you want to help spread the heat around, wrapping the bottom and sides of the tank with copper foil will work well. So, copper foil, then heater pad, then rigid insul, then tin foil, preferably with bubble wrap.