I haven't had any Aerogloss in a long time but I believe there is something different between it and Sig dope. Sig says to never use other products with Sig dope but that may just be their advertising to keep you buying only their products. I never tried mixing the two so can't say one way or the other if they're the same thing or not. But I also believe the Aerogloss thinner will probably work to glue your cowl together. Usually any of the "thinner" type products will work. I've even used acetone but it evaporates so fast it's harder to use as the glue. Doesn't stay liquid long enough to get a good "melt" between the 2 pieces.

One thing you could try is cut a piece out of the cowl (like a small scrap from where there will be an opening - like for the engine shaft or an intake hole). You don't have to be real accurate with cutting the hole (make sure it's in approximately the right spot, though) as you can enlarge it to the proper size later. You just want to get a small sample. Apply some of the Aerogloss thinner to the scrap piece and see how it acts. If it softens it and makes it "gummy" for a few seconds before drying, then it should work ok.