I mounted mine vertically to the back of the glove box. Had to section the glove box to make it shallower front to back, but I didn't care since the only thing I've ever kept in there is the reg and insurance cards and a few spare fuses. The nice thing about this is that it is out of the way, but easy to get at if it ever needs to be replaced (no need to drop the heater).
Oh, and those white plastic brackets are a dime a dozen at the junkyard from any domestic Ford of the EEC era. There are several different versions for different mounting positions, but they all fit the same computer chassis.