Tubus Smarti is made to fit on the end of the cantilever boss', and the fork eyelet.

their other front, low riders may need one of their fork-blade clamping mounts
but the mount-clamp is top-shelf-good.
quality bike racks go from one biker to another , strip them off when you sell the bike.

there are copies of the 70's Blackburn low rider racks.
the upper rear corner secured with a U bolt around the fork..

Your LBS may offer additional choices..