To further help with overtightend bolts, put the balsa filler end grain to the bolt head. Balsa has a lot higher compression strength in this direction at no increase in total weight. The thin ply is still a good suggestion as the sheeting is cross grain and it would be easy to pull through the sheeting by overtighenting the bolts.

A note to keep in mind related to the nylon bolts. They will stretch and stretch as you are tightening them. It is very easy to over tighten them, pre-stressing them so just an added amount of stress will cause them to fail. A rule of thumb I use is to finger tighten them, then using the wrench, go 1/2 turn more. This is enough friction to prevent the bolt from working out, but still leaving the bolt with all is tension strength avaiable.

Don