You could use more resin (although perhaps a stronger one than Easy Cast "tweaked" epoxy resin), and perhaps even create a mechanical hold of the resin *in/around* the slots/etc of the barrette finding instead of just an adhesive hold.
Or use a 2-part epoxy glue (often in twin plunger tubes) which is basically thick epoxy resin (the longer the setting time on an epoxy glue, the stronger it will generally be--so perhaps Devcon's 2 Ton 30-min setting one).
You might be able to use a permanent white glue meant for attaching metal to fabric (Jewel It, GemTac) if you weren't already using those. They will be slightly flexible (but strong) permanent white glues so may tolerate the flex of the finding, and again any time you can make the glue "go around" something the mechanical hold is good.
You might also try a completely mechanical hold by drilling a hole through part of the resin piece, or glueing on some tiny metal tubes or a bail or two, then running fine jewelry wire through those and in/around the barrette finding.
Or you could try using a piece of fabric, or something else thin that's almost as large as your resin piece, on the back side of the open barrette finding... then use one of the glues to hold that to the rein since you'll have a larger surface to glue it to.
By "alligator clips" do you mean the ones that are hinged at one end (not much stress there), or the kind of long triangular ones that snap and flex to close, or regular barrette findings?
alligator type hair clips:https://www.google.com/imagegs?q=alligator+clips+hair
regular barrette findings (can be short or long)...3 pieces if taken all the way aparthttp://zibbet-production.s3.amazonaws.com/images/230/1154853-original.jpg