You can get different size tubigrips, just find one that holds it in place without it being too tight.I have some Ervocom flags which are excellent.
Only issue I have is that the strap leaves a nasty rash mark afterwards. If I tighten it is sore if I loosen it comes loose.
Tried tubogrips which are too tight.
I think I need to do some experimenting.
Any ideas folks? Have a macron undershirt - only issue is that the pouches are quite flimsy.