Gift shop? Quality of the interior? Don't ask
me, I didn't get inside! When I visited
Fort Trumbull in New London, Connecticut,
it was also locked up tight, but its firing ports were merely covered with transparent (-ish) plastic, with several opportunities to snake an arm inside the fort holding a camera, but such opportunities are few (singular, in fact) at Fort Independence.
As mentioned, there were literally
hundreds of people out and about in the park, running, walking dogs, picnicking, and just generally enjoying the view of Boston Harbor. There was enough room for everybody, however, and very few managed to intrude on the pictures I took.