My cheap solution for towing in rain...
I can't recommend it, doing this every weekend, but 2 times a year it isn't any problem.
You need:
packaging foil (elastic)
some tape
right wrap-technique*
20min
*4 layers side by side front to rear (only one layer thick) / go around to fix the ends, couple times / important to cross once left rear to right front and opposite / don't wrap the mirrors / fix it with tape, that the tape pulls up the wrap on the sides a little...
I can't promise that it works every time, but it worked well in Hockenheim - just a question of the right technique.