if you are literally looking to put it on your rm (bicep) instead of wrist, it uses the same connectors as the standard Apple Watches, and there are a ton of arm straps for the Apple Watch. Just take a look at Amazon. I have a neoprene one that I got several years ago & it works great.
I have looked are the huge numbers of Apple Watch Armbands on Amazon for the U2 I have coming. I know what I like but some of them have little or no size adjustment. Some of them try to do an Otterbox Defender on the U2 making a rather large smart watch into a square hockey puck on your wrist.