Don Foster, Venturess II, #606
I have used 18-8 stainless machine screws and epoxy in situations like this. The screw just has to have enough thread to catch in the hole and hold the fixture flush against the cabin trunk. A little epoxy resin mixed with sawdust worked into the hole will fill in the voids and hold the screw fast.
When you want to remove the screw, just put an electric soldering iron on the head of the screw for 30 seconds, the epoxy softens and the screw will back out easily. A little marine caulk between the fixture and the cabin trunk will stop water intrusion.