I figured everything would be aluminum and a magnet won't stick...
Right, let me more specific than "slap"
I haven't actually installed one, but I have scouted the location. Take your seat off, drivers side below the oil dipstick. You'll see where the motor mount curves in under the side of the motor. It should stick pretty well there. Its also easy acsess to putting the cord for it up out of the back of the drivers seat, then run your extension cord in over the drivers door. That way you never take off with it still plugged in..... I have done that before on my pickup hahaha. If you plan to leave it in place there full time, I'd suggest securing it with something.
They also make a heat blanket, which would be easy enough to plop < --- (bad descriptor)err set on top or beside the motor. You could take that on and off, but I think personally I would permanent mount the magnetic and just leave the cord access behind my seat so I can unplug and go.
I know some people that use a tarp and a 100 watt droplight, but I feel like we can do better.
Also, I just sent you a link to the first one that came up. Please don't just order that one I linked, because I don't know its measurements.