I don't believe there is a 'right' answer for any heat issue, just different answers. Each keeper needs to over come the individual set up issues that they encounter. I also have a cool room, 68 is high and I often see 63 in the winter. Racks are very space efficient but are really really poor when it come to heat.

My solution may or may not work but it worked for me. I run cool side heaters on my rack, rigged up the 17 inch flexwatt on the cool side controlled by a herpstat it is set as you might expect to 82ºF and delivers 80 inside the tubs. The next thing was to add a piece of rigid foam insulation to the top and bottom of the rack to help a bit. I then (being a spaz with such things) stopped using the top and bottom tubs as they ran quite different from the others. I then wired the bottom with a 11 flexwatt strip the runs front to back (no animals in the bottom) and covered the front of that compartment so the heat holds and rises inside the rack. (it is on an on off t-stat set to 100º as it is just an air heater)

Since I have perfect temps but the effort to get there was quite silly really 3 t-stats for a 5 (3 as the top and bottom are useless) rack is a bit excessive I know. I have a very draughty place and heating up the whole room just isn't practical. The simple solution is if you want the rack just heat the room to the cool end temps. If that isn't possible well so other way needs to be found. Mine is a bit over the top but then again I have a 450 dollar thermometer as I just could not tolerate the error of cheap ones.

Good Luck maybe this might spark an idea.
Alex