The plastic is part of the boat. Basically, the top layer is the outside of the cockpit, the the bottom layer is the inside of the boat. Here is a not great pic of the traveler:
Traveler hauled to the end | Jim's Scampi 30 MK-IV Site - Helios
Its hard to see but its bolted down. The bolts go through the track, into the void them come out the bottom. The "bottom" is plastic just above the quarter birth.
I have no access to the void. It would be easier to fill if I did.