If there is a void, that's not a real good place for tightening down a bolt. Filling with epoxy might resist compression but I'm not sure.
Could you drill a an oversize hole and put a spacer in there flush to the headliner, then a washer before the nut?
Or get a small hole cutting bit like this style
Then cut through the headliner
only and insert a backing plate (or large washer) and then cover the hole with a bung cap. You can get bung caps in the "hard to find parts" drawer in the hardware section of Lowes. The drawers look like this...
and the caps look like this...
.
After you finish installing the traveler, cover the holes with the bung cap. You can also find these types of caps on Amazon. Do a google image search for "hole caps". West Marine even sells some intended to run a wire behind the headliner and give a port for the wire to exit.
There are a lot of choices. You just need to find one that is close in color and has the right hole size.