Build a fairface block wall across the length of a gallery room, from floor to ceiling, about 30m long, 1m or so away from the existing wall.
The visible side of the wall should be plastered smooth and painted brilliant white to match the gallery walls.
At either end will be a 1m gap between the false wall and the actual walls of the room, to allow access. Have staircases and galleries behind it to enable access to any part of the wall.
Drill a few dozen holes through the wall, about 130mm diameter, using a diamond tipped masonry hole-cutting bit.
Vary the height and spacing of the holes across the walls entire length.
Visitors can put their arm through a hole and stay for however long they like.