I can upload some pics for you tomorrow at school. The string is a stopper as well as a screw and nut on the stationary part of the launcher and on the slide that moves. You should be able to use one string throughout an entire tournament.
@Samuel Crouch (7842D) did you try 7842B’s approach? that is use spacers at the rear of the launcher to provide a “cushion” that absorbs some of the shock. we have yet to have a problem with it coming loose.
I’ve seen teams use a bunch of solid Teflon spacers in a row, foam on both sides, or lots of friction mat. The general idea of a stopper is to a.) distribute the force as evenly as possible along a large surface area and b.) provide a cushion of some sort so that the puncher doesn’t get damaged. For this reason, I would recommend foam.
To increase the fire rate, 3-4 high speed motors should do the trick. A high speed motor turns at a little over 2 revolutions per second, but of course this theoretical speed is decreased quite a bit by rubber bands. So, like @4361T said, direct driven should work fine. Depending on how much you reduce friction and how many motors you use, your fire rate will likely be around 0.5 - 0.7 seconds.