You can get the rep 20mm, however, that was made specifically for the 3rd gen PO. Withiin the endlinks of the rubber band, there are metal stubs in the middle that was designed for the PO case, since the case had holes for these stubs so the bad can be secured along with the spring bar. What I did, I cliped off those stubs and then used a little super glue to secure the band. No issues there. What I may do, when I have some funds, is go ahead and buy a gen rubber strap. Heck, my GMT is already modded with gen parts, why not get a gen strap, right?
Check out my GMT from the links in my signature.