Pillow ticking can be purchased at Walmart in the fabrics section.  Throw it in a load of laundry to wash the sizing out before you use it.  Make sure it is 100% cotton.  I rip it in 1 1/2 inch strips.  Lay the end of a strip across the muzzle, push a ball down to be flush with the muzzle, cut the patching off at the muzzle (which is what a patch knife is for) and ram the ball home.  I find this easier than trying to center the ball in precut patches.
If T-shirt material works for you, that's great.  However, you might buy a yard of pillow ticking and try that, see if it makes any difference.  Oh, Dixie Gunworks sells it also, at GREATLY inflated prices.  I'll stick with Wally World!
~WH~