Hmmmm 'Nards around us generally has a good selection of these fittings, they are generally kept separate from the other fitting for some odd reason, may have to look around a bit. Otherwise a good two part epoxy should hold up pretty well, just dope up both the inside and outside. Once you have the top off system setup is there going to be a great need to move it often? If yes then definately look into making a small bulkhead out of some fittings.