This is not a 3D print file, not Yet, maybe i print a valve for this, but i think this is usefull too. Print in A4 and use it fof cut velcro or heavy fabric. Join dotted part with paperclip or sew, add elastic or buckle.
If you found this useful, please donate what you think it is worth to my paypal.me. Help cover the time of design. Thanks, Enjoy!