Wurlitzer did glue the tubes into the tracker bars without nipples, but
I feel that's a pretty crummy way of doing it.  If you do, the only
thing I can think of that would work is shellac, and its bond to neo-
prene isn't very good.  It seems to have worked with natural rubber
(which is what Wurlitzer used).

When I do these things I drill out the holes slightly and glue in wooden
dowels, then sand down the back until the dowels are even with the
original wood.  Then I drill the dowels for 5/32-inch nipples and
install them with shellac.  I've never had a bit of trouble with this
method.

Best,

Mike Kitner