Burn hole in backpack

Hello Hans, try using a Sikaflex rubber silicone and stick a patch from inside the backpack , this would assure weatherproofing. or use aquaseal as a repair for wetsuits , there are also a sticky tape for these king of materials , but it needs to be ironed from inside , so you need to check what temperature it can survive, usually not above 70-80 deg C. ,thats if you rely on your backpack to be weatherproof / waterproof, otherwise just sew it together , . and don’t worry how it looks, most people would admire your excellent backpack rather than look for faults in it, you can always say it was an adventure accident like a splashing lava rather than a cigarette burn.