Keep in mind, I did not see a tank average increase of 4mpg, just the morning commute averages improved by that much with the heaters and this is in sub freezing temps, the benefit decreases as the temps rise, however I use the heaters year round
Don't sweat the voids too much, I left a decent sized void on my trans pan and it did not hurt anything. The conductors in the pad could overheat, so it might be nice to put in some epoxy, but most epoxys are insulators so they won't transmit the heat to the pan.