I just made a set of engine cover gaskets out of an old car inner tube. Not complete yet, still need to punch the bolt holes, but this is what I have done so far.
First get an old car tube and cut large enough sections out of it. Since the tube won't actually lay flat, I used some plyboard to stretch them out as flat as I could, and using a sharpie, I traced the design onto the tube pieces, using either the old gaskets or the covers themselves as the templates.
Once you have traced the inside and outside of the gasket, trace all bolt holes and oil passages. Then cut them out using scissors, be very careful and only take small bites out of the material so you don't over cut and have to start again.
Home made gasket on the right
Home made gasket on the right
Home made gasket setting on engine cover right side
Home made gasket setting on engine cover left side
Both covers with home made gaskets setting on them
Where I cut them out of
Like I said, I still need to punch the bolt holes.