I came up with a inexpensive and simple-to-make boring setup for deep vessels.
The shaft is a 1″ galvanized pipe reduced to fit a brass compression fitting.
The brass compression fitting holds a 5/8 boring bar (Grizzly H2718) a with a 1/4″ square cutting bit (Grizzly G1507) that as been ground for vessel cutting.
The boring bar can be extended, retracted or rotated by loosening the compression fitting.
The bit can be adjusted with a set screw.
Under testing (see video channel) the bit and bar held solid and the large diameter pipe reduced vibration when extended 12-24 inches over the tool rest.
I attached the handle from my Ryobi grinder to the main shaft using a electrical grounding clamp (Lowes).