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Well its possible but IMO not worth it..
Lets see, the commercial unit says 2-4 watts of power in their design which should be enough to overheat the secondary if continuously on.
Ohms law says R=V^2/P
So for 12v , 144/2watts = 72 ohms worth of resistors.
Resistors come in standard values 10,12,15,18,20,22,27,33,47,56,68... .
4x18ohm resistors would theoretically do it, but in 1/2 watt format they will cook if their generated heat cant be adequately transferred to the mirror.
Personally I think its a bad idea to go down this path and have heat plumes streaming off your secondary mirror when such a cheap commercial solution will do it right and regulate the mirror temp to be a degree above ambient.
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