Anyone come across a source for a *leak proof* bulkhead fitting for
standard 68mm rainwater pipe?

I am fitting out and old galvanised tank for rainwater harvesting (first
job, glass fibre the inside to hide the rust) and want to arrange an
overflow using a diverter coupler which means the fill pipe will be
below water level before the overflow level is reached.

Water gathered from a 55' x 18' roof so I need the full 68mm size!
--
Tim Lamb