Craft foam, or epoxy putty is what I would use. You can get a small bit of epoxy putty from pretty much any hardware store. It is just as moldable as clay but cures really strong. It should stick to that surface if you rough it up enough.
Do you have a picture of the other side of your arm bracer so I can see how you attached the new straps? I got those same bracers but need to attach new straps for a different cosplay. Thanks
