I really havent used heat cable, but I have heard good things about it.
Is the mini-fridge the smaller square one or the slightly bigger rectangular one?
The flexwatt will work very well, but you need to solder a plug onto the flexwatt. An old 2-prong extension cord works well for this. You cut off the receptical end (leaving the plug end intact), you then seperate the two wires up about eight inches, then strip-off 3/4" of the sheathing om each to expose the copper wire inside. This is what you attach to the Flexwatt.
Down the sides of the Flexwatt you have copper "rails" - one on each side. The heating element is the insulated wire mesh that runs in a zig-zag pattern inbetween the two "rails". You expose a small portion at the corner (both on the same end) of each "rail" by scratching off the plastic coating with a razor or a small sharp knife. You then have to solder one lead from the extension cord onto each one of the exposed "rail" ends. After the wires are secured, you need to cover the exposed solder and copper wire with electricians tape or a insulated glue works also.
I would use 11" Flexwatt because you wont need such a long piece. I would cover the bottom of the fridge (it can even run up the sides a bit), securing the Flexwatt in place with heat tape or even high temp / hot glue.
If you dont want to solder the Flexwatt or dont have access to a soldering iron or whatever? You can by a Under the Tank heater I think? I am not sure if the newer ones get as hot though...you will need to check? My older ones get very hot. I thought I read somewhere that the newer ones do not get as hot, but I am not sure? The manufacturers might have started limiting the heat output, they will put a current limiting resitor in series with the plug, you can probably remove it but you would need a soldering iron again to do this??
The most important thing is that you use a really good controller set for 89 - 90 degrees. I use Herpstat, but Helix is supposed to be excellent as well. This is imperative to maintain temps precisely at what you set it for.
Also, you want to use an egg box to maintain humidity. This is just a tupperware box, large enough to hold the eggs. You poke a couple hole in the sides, then put wet vermiculite 3/4" on the bottom and place 1 or 2 pieces of eggcrate (the light fixture kind) ontop of the vermiculite. The eggs are then placed on top of the eggcrate, so they do not touch the wet vermiculite and die.
This is good for now, let me know if you get stuck or have additional questions.