M.D. is a fine machinest and has made a really nice flash diverter. Unfortunately, these diverters are not on the market and unless you can make one, you will just have to learn how to deal with the cap spray. I have tried a regular flash diverter (cup) and that was a no go because I could not get the cap onto the nipple with the cup in place. I love my underhammer so I came up with a solution. I made an arm sleeve of canvas that protects my arm from the spray. It works grea. It is just a canvas tube with elastic at the upper end to keep it in place. It's extremely easy to make. I have also tried changing how I hold my underhammer. It is a very light weight gun (H&A Buggy Rifle) so I can, if I wish, use a grip just as if I was shooting a handgun. My right hand grips the gun in the usual fashion and my left hand goes directly over my right hand. Sounds awkward but with my light weight rifle that position is really quite comfortable and quite stable and my arm is no longer in the way of any cap spray.
As for the cap staying in place, that is not a problem if you use the correct nipple for the cap you are using. Usually, you will be shooting #11 caps so if your nipple is a #11 nipple, there will be no problem with the cap falling off. Putting the cap on is not a problem, as M.D. said, just turn your rifle on its side and it is quite easy to put your cap on. I use my finger to press that cap on as tight as it will go after putting it on the nipple. Properly applied, it will stay in place anyway but giving it that little press with my finger is just sort of a "belt and suspenders" thing with me.
If you want an underhammer, don't let the cap spray problem prevent you from buying it. There are many solutions to the problem. If you are a machinest like M.D., you can make a good flash diverter like his. It's a dandy. If not, you can look for someone to make one for you or just make yourself an arm sleeve like I did. I wouldn't part with my little .45 cal. H&A Buggy Rifle.