clear nitrocellulose lacquer. Basically the same as NITRATE dope... Nitrate doesn't shrink very much so the fabric will have to be taught before you apply it. You can make your own BUTYRATE dope (shrinks more) if you can get butyrate sheets, grind them up (a blender works well for this)into a coarse powder, and soak them in a GLASS or METAL container in lacquer thinner. They will dissolve. You need about 1 part butyrate to 2 or 3 parts thinner. It will take several days for it to dissolve. Keep it tightly closed.

If you can find some material that is heat shrinkable (such a dacron coat lining or a daccron aircaft covering such as Sig Koverall, Stits Lite, or Aircraft Spruce uncertified dacron) you can apply it with water-based polyurethane (clear). There are MANY articles here on that process.