Seeking ideas to repair a B72 steel frame rail. Damage was caused by using a regular 2 rail seatpost.

A butt weld repair has failed, as I thought it would.

A replacement frame from Brooks was found to be about 30 mm too short for the 40 yo well broken in cover.

I’d like to braze a 5mm ID steel sleeve over the break. Any ideas?

Original B72 chrome frame with broken RHS rail in foreground, shorter B72 black powder frame behind with original cover.