[sc_rufctr]

Wrap the spare seat in cling wrap and make a mold using polyester resin backed by a single layer of fiber glass.
Pop it off when dry and then wax it and make your cover using it as a mold.
You could build up the spare seat a little so that the cover fits the good seat properly.

You could also form ABS into your new mold using a heat gun as suggested but using an oven to get it uniformly soft may work better.
Depending on the ABS but using an oven gives you about 20 to 30 seconds of working time.
You'll need some heat resistant gloves. There are "Yellow" industrial gloves that are designed to insulate your hands from heat.
I think they're made from a Kevlar material. (not cheap)

There are probably about 10 different ways to do this... Have fun with it.