If it's just a crack and it still fits tight, cyanoacrylate adhesives work great (super-glue). If you are wanting to reinforce it with another layer or something, JB weld or another 5-minute type epoxy will work well. If the epoxy seems too thick, SLIGHTLY warm it up with a heat gun or hair dryer, and it will thin out significantly and flow into the nooks and crannies much better.
Phil