Hold on.

What plane is it?. Usually all you have to do is cut small 1/16 or 3/32 thick plywood squares about 1/4" -3/8" square and CA them to the backside of the balsa. Then you put the canopy in place, screw in the screws. Once you get the canopy installed completely remove all of the screws and then place a drop of CA in the holes. The CA will dry and harden the threads in the plywood. This will last forever. Another thing I like to do that will really help is to cut a piece of fuel tubing about 1/16" or a smidge thicker and place a washer and then the fuel tubing over the screw before you screw it in. This acts as a vibration damper and prevents the screw from wallowing out the cowl. Sometimes you can get away without doing this but it is a VERY effective way to ensure that vibration doe not ruin your cowl by wallowing out the holes until the screw heads are smaller than the holes.

Another option to the plywood squares, and a very good one is blind nuts. You install the blind nuts backwards with a drop of epoxy on the inside of the cowl after you drill the holes. Then its just a matter of using a socket head or button head bolt with a lockwasher, washer and piece of fuel tubing. Looks sortof like this. .

http://www.microfasteners.com/catalo...cts/SSCSBA.cfm
http://www.microfasteners.com/catalo...cts/SSCSCA.cfm
http://www.microfasteners.com/catalo...cts/WASFWK.cfm
http://www.microfasteners.com/catalo...ucts/NUTBN.cfm


By the way... EVERYONE should know about Microfasteners. You can order nuts bolts and washers 100 at a time for the cost of a few from "other" places. For instance, 4/40 socket head cap screws are $2 for about 6 from the typical LHS, from MF you can get 100 for <$5.