I just fitted both my pumps to a bit of ply and wedged it on the floor, between the tank and the van wall. Works fine, and when it's due to get below -4 C, I take the pumps indoors; very easy since it's mounted on ply and I've attached a small plug to the wires which means I just unplug it, undo a jubilee clip and the whole shebang comes in side.
It doesn't look very pretty, but it's 100% functional.