Euphoniums and piston valve tubas can collect water in the bottom of the valves, which will drip on your legs when you tip the horn slightly. Besson used to make a metal water catcher that snaps onto the nipples on the bottom valve caps. However, I found it awkward to use and hard to slide on and off quietly. So I decided to make my own.
You need some plactic tubing from a hardware department, probably about 4-7/8" long and about 1/2" to 3/4" in diameter. I use the semi-clear vinyl tubing, but the gray waterpipe-type tubing will also work. It needs to be at least a little flexible. Then you cut a slot lengthwise down the middle, about 3" long. You can use corks or rubber stoppers to cap the open ends of the tube. It will easily snap on the valve cap nipples and will hold a lot of water before it needs to be emptied.