I had the canopy come off an almost 30 year old 25 size pattern the other day. When I cleaned it with 409 and alcohol it became deformed, the bottom shrunk together. The canopy was 9" but found I could put it over an 11" canopy to reshape it but how to soften it. I tried a heat gun but it wouldn't retain the new shape. Finally I filled the old canopy with water from a hot water dispenser, about 195° F. and put it over the over canopy. I guess filling it with hot water gave a more even distrubition of temperature and that worked.
No chanch of melting the canopy either.