On 25/04/2019 14:19, T i m wrote:

a 3D printed 'sleeve' would be basically a square box section, about
40 mm long with ~1.5mm thick walls and rounded corners. It would want
to be a close interference fit on the inner section tube (and possibly
glued / pinned into place) and a light sliding fit on the inside of
the larger section tube.

Ideas?

My 2p worth is simply to build up a thin fibreglass sleeve where you
want it on the inner tube. It's robust and easy to shape.

Fibreglass won't be at all easy to remove if you do eventually make
something on the 3D printer, but OTOH it will last indefinitely, so you
won't need to make anything on the 3D printer.