What kind of epoxy did you use? That tube stuff is crap, you have to use a real liquid epoxy resin. I build boards for myself and a few friends, and I have to say I've never seen "inserts" like that. Those are a joke! (what kind of board?) If you have another board to ride for a while and can wait, go order some real inserts from www.snowboardmaterials.com
(look under the kiteboard parts section). Those inserts come with a thin cover over top to keep the resin out, have a closed bottom, and a wide base. The normal procedure is to put them in with resin and some chopped glass or filler in the resin, then they get glassed over and then drilled out with a countersink bit.
If you do try to do another fix yourself, you might pack those crappy "inserts" with playdough to keep out the resin, and after gluing them in with real resin, glass over top with a couple patches a couple inches in diameter. Make sure you sand the area where you will be glassing over with some coarse sandpaper to improve the bond. Most of the ugly repair should remain inder the strap.