Originally Posted by
fietsbob
How about , Plywood,say 3/8", 10mm thick , ?
and P clamps, plastic should do , with enough of them used.
and some screws into the wood, or bolts thru it.
I widened a narrow front rack , in this manner.
Depending on the mass of the OP's duffel bag, the same could be done using coroplast without adding too much weight...