Inside my GGFs 1880 2 1/2 row D/G is a section of a playing card .. cut to a strip about 1" wide and creased then glued over the worn/torn fold... very cool.
Gaffers tape might be a good choice. It is quite strong, like duct tape, but it doesn't leave a gunky mess like duct tape does if you try to remove it.
I have used gaffers tape for internal attaching and sealing of different things.