I have never heard of anyone trying a donut spare on a 968. I would advise against it for a number of reasons. First, the smaller diameter would result in an already low car being dangerously lower on whatever corner you put the spare. Second, the narrow tread combined with the changed ride height would make the car squirrelly at any speed above walking. Finally, where do you plan to find a 5x130 donut in the proper offset? Sourcing and stowing such an animal might prove to be impossible.
I would suggest getting one off of an older Boxster or 944. There are lots of them in junk yards, and picking up an aftermarket compressor to inflate the tire should the need ever arise.