The description relates to devices that can include a hollow arm that has an open side and a cover engaging region around the open side. The cover engaging region defines a first width at the open side, a second width farther from the open side, and a third width that is farther from the open side than the first width and the second width. The device can also include an arm cover that includes an arm engaging region. The arm engaging region can define a fourth width at the open side, a fifth width farther from the open side, and a sixth width that is farther from the open side than the fourth width and the fifth width. The sixth width can be greater than the fifth width and the second width and less than the first width and the fourth width.