The Star Wars: Force Awakens official Novelisation, Star Wars: Force Awakens junior novelisation and original script all identify this mystery structure as a wall.
One such individual clung insectlike to a dark metal wall pimpled with
protruding sensors, manipulators, and other decaying mechanisms. Clad
in light protective goggles with green lenses, face mask, gloves, and
gray desert clothing, the busy figure was burdened with a substantial
backpack. A multifunction staff strapped to her back made precision
work in such tight and dangerous quarters difficult. Wielding an
assortment of tools, the scavenger was excising an assortment of small
devices from one metal wall. One after another, bits of booty found
their way into the satchel that hung below the slender figure.
Star Wars: Force Awakens - Official Novelisation
She hung on to the wall of a demolished destroyer and used her tools to wiggle free various components that rust hadn’t devoured. The ship was a treasure trove of metal sheeting and military-grade tech that, if given a proper cleaning, could fetch Rey a meal or two at Niima Outpost.
Star Wars: Force Awakens - Junior Novelisation
A metal sheet is pulled open to reveal the wrapped up face of A SCAVENGER, perhaps alien, in GOGGLES, FACE MASK and GLOVES. Backpack with a STAFF strapped to it. Uses TOOLS to remove various MECHANISMS from inside the wall. We are in an upside-down, canted CORRIDOR. The Scavenger finds a valuable piece, drops it in a SATCHEL. Swings the bag back and begins CLIMBING DOWN on a cable, between treacherous WALLS OF MACHINERY, headed to:
Star Wars: Force Awakens - Script