I have one that I got from Track of the Wolf. It works, but you have to be careful to get it centered over the flash hole so there's no leakage. I pull the lock, stuff a paper towel into the lock mortise to absorb any leakage, then screw the clamp down. (The seal is via a little rubber o-ring on the flusher.) Before actually putting water down the bore, I put my thumb over the muzzle and blow into the hose to check for air leaks. If air can't get through, neither can water. If okay, I put some water down the barrel, let the breech soak for a few minutes, & dump it. Then a few patches pumping with the hose end in a small pail. Dump that fouled water. Do it again with clean water. Take hose off. Run patches through bore until dry. Oil.
Cleanup usually takes me 15 minutes or so.
If I didn't use the flush hose, rather than removing a pinned barrel for cleaning, I'd shove a toothpick or wood match stem into the vent to seal it and run wet patches up/down, and maybe use a breech scraper.