I ordered the 12.5 mm sail stops - might be an issue because the screws holding the channel onto the camper are not counter-sunk. That's the reason I had the local welder give me an eighth inch flat on the side of the round plug opposite the hook weld.
When I get the 12.5 mm plastic plugs, I'll try them and let you all know. If you used the 3/8" sail plugs they would probably fit in the channel and slide past the channel mounting screws.
A sanding wheel would probably flatten one surface of the plastic plugs to allow them to slide past the channel mounting screws.
I suspect all of these solutions work - I'm not nautical (canoe with sail is as close as I ever got!) - so was not aware of something like the sail plugs - would have saved me the $180 I spend on the stainless plug/hook, but I'll have them forever......
I'm thinking some bubble gum stuck at the ends of the channel will keep the plugs from sliding out - or maybe an 1/8" bolt slipped into a bored hole at each end .......