Pattex/Loctite Ultra Gel, likely available under different brand names as well just check the spec and compare, very strong bond and easy to use
Now that I have tried to fix over 1,000 backpacks onto 1:32 Hat figures I have some feedback; 1. I don’t like most of the Hat backpack configurations. The Longcoat sets are the WORST; the peg often does not fit into backpack opening at all. However, once afixed, this typically stay together.
2. Much prefer the cartrage pouch separate from backpack (as done on Voltigeurs & Carabiniers) and afixed to figure. Avoids complications involved when cartrage box is attached to backpack.
3. There has to be a better backpack slotting arrangement; that of the Hat French light sets leaves too much of a gap and often results in the backpack popping off the back as the glue dries.
4. The recommended glue worked better than other options I tried, but it was still not great and required a ton of glue per backpack, with still mixed results.