I'm assuming that you have an "L" shaped rail that fastens to the deck on each side of the side and coverrs the lip of the sliding hatch on each side.
Yes, this is correct. Then, there are hand rails fastened just above this. The rail above the hatch (the "L" shaped one) is fastened with two screws into the deck, with nuts underneath, in the cabin. Previous to my refurbishment, the previous owner had five bolts going through the tops of the handrail at different locations, also fastened to the underside of the deck via washers and bolts. The previous owner then put a strip of pine that runs the length of the rails on each side (starboard and port), and carved indentations into the pine on the topside, so the nuts/bolts could sit in the indentations, and fastened this (via hot glue, from a glue gun, of all things) to the underside of the deck).
I modified this so that the longer bolts, the ones coming from the handrails, are actually about 1" longer, and now run through the pine strip on the underside of the deck, and are fastened underneath. My guess is that this originally was because the previous owner didn't seal the deck hardware properly, and was getting water from the screw holes dripping down when it rained, as they are right above the quarter berths. It could also have been just for safety, so he/she didn't scrape their head on the screws on the ceiling when waking up. In either case, I'm now simply using the pine strips to distribute the pressure a bit more across the length of the rails from underneath.
Consider the use of two thin (ca. 1/16th inch) polycoarbonate strips. One wider strip would be on the bottom and fastened under the rail and the slide. The second, upper strip, would be positioned only under the rail. This would give you a new slick surface and an added 1/16th increase in the gap.
Where would I get something like this?
An added check may be available. Do you have a hasp for a lock that attaches the slide to a fixed point? Does this hasp for the lock close without rasing the slide for a proper fit? This hasp position may indicate something that has changed.
I'm not sure what you're talking about here. Where would this hasp be located? I don't think I have this, but I could be wrong.