External heart rate monitor for Apple watch

Does anybody know of either a chest strap for Apple Watch or an external heart rate monitor that is compatible with it? From what I’ve read online there are several externals that can work alongside the watch, however, it isn’t clear if they actually add to your rings or not. The problem I have is that I do combat sports so I can’t comfortably or safely wear my watch when training but this means I lose a lot of progress on my movement and exercise rings. When I bought the watch the sales assistant mentioned chest straps but then I haven’t been able to find anything that fits what I need. Any advice appreciated, thanks