Back in 75 fell on ice and ruined pair of Bates bags on 73 GT550. Replaced with Harley bags and mounts (then cheap).
Know or find welding metal shop willing to help.
Harley used solid bar stock in L shaped loop. Short L loop fitted (spring) C channel riveted to bag bottom. Btw L uppers a flat supported DZUSE fasteners clipped inside box.
Appriate mounts welded to loop.