It's a while since I've used 'Maskol' (must get some more), but it does work well. The good thing about canopy masking with it (most, anyway), is thatyou can coat the whole canopy, then just cut-out and remove the frame areas. Paint, let it dry, then peel off the 'Maskol'. Simple!
If you can't get Maskol', or a similar product, you can use PVA, only just slightly thinned with water. But try it on a piece of scrap first, to check it's not too thinned. Use in the same way as 'Maskol'.