Here's my method.
Get PL polyurethane premium construction adhesive. Also get a hot-melt glue gun and sufficient stash of glue sticks.
Spread construction adhesive on either surface, ensolite or
fiberglass. But leave some spots without glue. Then fire up your hot-melt glue gun and one at a time put a small dollop on a bare spot and quickly push the ensolite into position until the hot glue cools. Continue with this sequence until it is all up.
The reason for the hot melt glue is to hold the ensolite in place until the construction adhesive sets up.
Good luck.