Last weekend I went to the gun show in Valley Forge and found a 1945 vintage USGI aluminum canteen and carrier for $25. (I remember when they were a few bucks.) I like the old metal canteens because unlike plastic vessels you can heat water in them. Plus, compared with plastic canteens, they're just cool.
The canteen and cap were in good shape but the cork gasket was missing. So, I went to Lowe's in search of a replacement. I'd read of using a silicone end cap for for pipes as a source for gasket material, but couldn't find any in stock. Naturally, the store staff was nowhere to be found.
I did find over in the adhesive section some DAP food/aquarium grade sealant/adhesive. (The link is to Grainger.) I put a layer a couple millimeters thick inside the top of the cap, being careful to keep it off the threads. Then I let the goop cure for a couple of days.
Tonight I filled the canteen with water, screwed on the cap, and shook it vigorously. It didn't leak, so I'm going to chalk up this experiment as a success.
One mod I may make is to attach the cap with a snap link to the body of the canteen. That will allow me to remove the plastic cap and place the canteen over a fire.
A cheaper alternative to the USGI aluminum canteens are the surplus French aluminum canteens. They appear to have a wider mouth than the US canteens, which could be handy if you wanted to put ice cubes in it, for example.