RE: how to sttrengthen without adding weight?
What Bob said +2. I fly a Goldwing 30 sized Yak 55 and other than saturating the glue joints with thin CA, reinforcing the firewall with aluminum corner brackets, and adding a few pieces of wood to the LG area, it has held together quite well over 2 seasons of hard flying.
Regarding aliphatic resin glues, I agree they are great glues but not for exterior application where little of the glue can penetrate the joint.