When I lost my muffler's endcap, I went back to retrieve it, and watched as a car ran it over. one thing you can do is to find a big fender washer, one that is big enough to cover the end of the exhaust pipe, then dome it with a ball peen hammer and another even larger washer, one with a center hole that is a little smaller than the outer diameter of the washer you intend to dome. To do this place the largest washer on a bench, or the anvil part of a vice, then take the washer you are going to dome, and center it so that it is concentric to the larger washer, next, you hit the washer with the ball end of the hammer. Once the washer is domed, you drill one or two holes for the exhaust to escape from. Then attach the washer to the muffler, so that the end is convex.
I set my bike up this way and have had no problems with it. It is a lot cheaper than buying a new muffler just for the sake of cannibalizing the end cap.