Just about any epoxy will do. I'd use a 4:1 epoxy like AcraGlas thickened with chopped mat or microballoons. Putty-like epoxies could also work, but I don't have as much experience with them other than noticing they seem to cost a lot more.

You'll want to keep the barrel floating, so be sure to put a few layers of tape on it and then hit it really well with release agent. Presto: custom barrel channel.