I (probably like most) have tried various things when I don't have the right gear.  I thought it might be good (if we can stand the humiliation) to post em up so everyone can see what works and what doesn't.  I'll start.

I've tried covering a plug in vinyl to give a good surface as I was too lazy to seal it and paint it properly.  I tried airelease vinyl that had several coats of wax on it and I got away with it.  Yay!  I tried a cheaper vinyl and it wrinkles like mad.  I tried the airelease with fewer wax coats and got a slight bubbling.  Worked once but probably not something I'll chance again.

I used 'antique pine' wax once when I'd run out of proper mould release.