We normally replace lead tubing with neoprene tubing.  We seal it to
the stack with automotive silicone windshield sealant.  This is a low
viscosity material and also acts as a lubricant, allowing easier insertion
into the hole.  It should also work when putting lead on tracker bars.

Shellac has also been mentioned here as a sealant.  Purchase a bag of dark
button shellac, and crush a couple of buttons and mix with sufficient
alcohol to form a thick paste.  We always keep a container of this handy
(in a baby food jar or similar sized jar) to use as glue for gaskets and
felts, etc.

Bob Lemon