You could use fusible interfacing, which would basically glue a very thin layer of stiffer fabric to the T-shirt fabric. That would make the T-shirt fabric easier to work with, and more fit for purpose as a cap fabric.
I haven't done this before with old T-shirts, so I can't guarantee the finish, but it should work, theoretically.
If you're very keen to give it a go, let's chat in PMs to arrange things?
You could use fusible interfacing, which would basically glue a very thin layer of stiffer fabric to the T-shirt fabric. That would make the T-shirt fabric easier to work with, and more fit for purpose as a cap fabric.
I haven't done this before with old T-shirts, so I can't guarantee the finish, but it should work, theoretically.
If you're very keen to give it a go, let's chat in PMs to arrange things?