carbon mast

Glue is almost always better than mechanical fasteners - the exception is if the part has to be removable.
If you have to use mechanical fasteners always use bolts rather than screws or rivets.

This is the sort that sockets into the top of the mast is it? If it is a socket type it might be an occasion for a bolt - just a very thin one right through the lot to keep it in place. Obviously needs to be located to miss the halyard which can be tricky. Otherwise something like an impact adhesive might be better than epoxy. This is assuming the glue is to keep it in place rather than take load.

Does it need fastening at all? my old 700 had its plug style sheave just pushed in, if you are worried about it being too loose wrap a bit of parcel tape round it to pack it out a bit then just push it into place.  No drilling holes, and no epoxy or glue clogging it all up and making it hard to remove

It should be fine, at the end of the day if it comes out you arent going to lose it because there is a halyard through it so it wont be a disaster.  If it does ever fall out then maybe drill and bolt it but i would suggest that until then it would be easier and probably better for the mast and the fitting just to pack it with parcel tape.  Parcel tape is the tape of choice for loose laser mast sections as it is very thin so it is easier to get it to fit right, this is why i chose it for the job you are doing.  It also doesnt leave any sticky gunk in the mast or in the fitting and seems to be harder wearing than pvc tape which is too soft and just peels out of the way when you push the fitting in.

You could stick it in with hot glue. (I used to use it for fixing the handles and sleeves onto carbon oar looms).