having trouble understanding the deck construction.
First of all, I would send a Private Message to
@Crazy Dave Condon. He has extensive knowledge of these boats.
The construction of most boat cabins involves a sandwich of fiberglass-plywood-fiberglass then in the 27's case it appears to be fiberglass interior surface. Most designs have the cabin tops lightly constructed to minimize weight.
If you are considering a traveler in that location, as in the image, you likely are going to need the bolts and large backing plate as shown in the above picture. The deck in that part of your boat may not have been designed to handle the stresses of a mainsheet traveler with just bolts and fender washers.
Here is a bit of background on the 27.
Hunter 27 - Practical Sailor
Here is one way that the designer spreads the stresses on the cabin top.