I broke the watch strap clip on an aftermarket silicone band on my Garmin and it was annoying sliding the thing under the inside of my wrist so I printed a replacement one based upon the geometry of the band itself (not the old clip). Mine was printed in PLA and works just fine, but if you want an easier installation, I would recommend a flexible material. To install it, I had to unscrew the band from the watch using the funky torques screwdrivers that came with the replacement band and then put it on the band before reattaching it. This will probably be different with a flexible, but I'm not sure. 

After months of use, I have found that this is significantly more robust than the original design (i.e. this one clip has been what has broke on 2 of my previous bands) and has held up well even in the high-impact warehouse where I work. I'm not sure how safe this is for your skin, but I have not had an issue, though I would recommend cleaning it regularly. If you're concerned about that, this 15-minute fix should hold you over on runs until your new band arrives. Trust me, I get it.