To permanently affix the deck to the trailer I used 3/8″ stainless steel carriage bolts. Small pilot holes are drilled from underneath the deck through the bolt holes in the trailer. From there the holes are widened from the top so the carriage bolt heads to be countersunk and not exposed in the cabin.
Stainless steel carriage bolts, fender washers, lock nuts, and nylock nuts.
I found that I needed torque washers, after trying the initial tightening of the first bolt I managed to get it to spin out of its square hole. Unfortunately the torque washers are zinc so to get around any problematic metal interaction with the stainless steel I will coat the underside of the heads and upper threads with tar. As an unintended benefit it will help to seal the holes.
Installed the bolts with silicone between the frame and deck around the hole to help with the seal