I'm with you Kelsey - the caps absolutely suck and I've not found way around it except to push the lig as far back on the piece as possible to get the cap to grip a little better. This might be a pain, but wrapping a rubber band around it will help keep it on. My solution is to put the mouthpiece in a small zippered case that is just big enough to fit the mouthpiece inside of it - then the cap doesn't have room to fall off.

I used a regular Rovner plastic cap and cut it so that the side pieces slid under the ligature wires (for want of a more descriptive term) and it always held tight and protected my reed(s). It's hard to explain without showing a pic of how I cut and trimmed the cap, but I ended up selling the lig and cap with a BARI metal tenor mpc on e-Bay . The guy was amazed at how I figured out how to do it. I used to keep digital pics of all the mouthpieces, ligs and caps I owned, but my last computer's hard drive crashed will all the pics. I should've backed it up. Yes, I one of those people who've gone through dozens (read 52) mouthpieces since I started playing more than 30 years ago. And I at least kept a listing of all of them. Geez, could've purchased a new sax with all that money spent.