Charles Lewton-Brain sells a book made to order for solving this
problem. The Recycling, Use, & Repair of Tools - Alexander Weygers
This is a great book I highly recommend it.
Unusual Mandrels in a triangular cross section can be made from old
triangular files. Get thee to a flea market or junkyard. Old files
can be found up to 12 " long. Find some one to anneal it and sand
the sides flush and smooth.
The recycling, use, and repair of tools
By Alexander G Weygers
Manufacturer: Van Nostrand Reinhold Co
Release data: 1978