I actually sourced one thanks to a clever recommendation from Jason S. The Nitto lamp mount/lamp holder comes with one of the daruma compression nut-bolts. The lamp mount is
$22 from Analog Cycles, so $7 more than the regular replacement bolt from Riv. But you also get a cool light mount bracket you could use on nearly any rack with eyelet mounts. And more importantly, the light mounts are readily available from both Analog and Riv and probably other sources.
Thanks for the offer David! I've already put in the order w/Analog so I'm good to go.