Could try something with a "hose-clamp" type of mount, I think Blackburn Mars uses them. (at least the separate replacement mount you can buy is) Strap it around the "hand grab" on top.

A velcro strap should be able to secure a belt-clip type light on either side of the grab bar - run the strap between the hole under the grab and the hole at either "ear".

And don't forget the big ol' run of red DOT conspicuity tape on the back of the seat!