Mask for round canopy frame..

I've found the easiest way to mask ANY canopy is to use Bare Metal Foil (and no, this is not a paid advertisement).
Just cut a piece of foil a little larger than the window you want to mask, burnish it down onto the canopy frame with a toothpick, then LIGHTLY trace the frame with a new number 11 X-Acto blade.
Carefully remove the foil from the frame, then re-burnish the foil onto the "glass" to prevent paint creeping under the foil.
Then just paint and remove the mask when it's dry.
It's easy and fool proof.
Good luck.

Or you can use a template with circles of various sizes, the more sizes, the better. Gauge the sizes for the inner and outer diameters by superimposing the template on the canopy frame and finding the nearest-sized circles for the frame dimensions. Trace the 2 circles on masking tape and cut them out: a solid circle mask for the inner diameter and a hole mask for the outer one. Then place on canopy. Good luck.

There is a couple ways you can try.

One way is there are products called foto frisket or parafilm. They are clear. You can take either one, stick it to the canopy. Then cut the mask carefully out with an x-acto knife following the frame of the canopy.

You can also take Tamiya tape and do the same procedure as above after burishing the tape down to the canopy. Tamiya tape is thin enough to see the frame to cut it out.

If using blue tape or regular masking tape. You can measure the size of the circle with a pair of calipers or a childs compass ( the ones to draw circles). Then draw the circle on the tape. Cut out the circle. The place it on the canopy.

Hope this helps

Paul