there is an indent on the other end thats how it holds in the magazine. Its not the circle end its other. if you do the strap tight enough it wont fall out.
http://www.airsoftatlanta.com/images/tm_p90_locap_jpg.jpg
Look at the far end of the picture of the P90 Magazine thats what im talking about. The two sides poking out you can put the strap in middle of those and it wont fall out.