I’ve done a test print of just the attachment between the arm and the frame, to check that everything lines up and fits properly; it works beautifully.
There are a couple of small problems that this has revealed with the design. First, the screw won’t physically fit underneath the attachment for the bottom mounting, let alone a screwdriver: something that I overlooked. The same will be true for the left and right sides when it is part of the bigger design for the frame. So I’ve moved the countersinks (for the screw head) to the inside on all but the top hole, in the CAD software, so that the screws can actually be screwed in properly.
Secondly, that same small gap meant that I couldn’t properly remove the support material because my pliers won’t fit:
Thankfully in the actual design the wall will be a lot taller so this won’t be a problem.
Finally, as can be seen in the top picture, the arm and the fitting don’t quite line up perfectly: there’s some unwanted space. This is primarily because I created the two at very slightly different sizes (the arm is 0.5mm smaller than the gap, for the tolerance), and then filleted the edges on both with a 10mm radius: this means that the edges don’t quite line up. I’ll get around this by just making an offset of the fitting shape and using that for the arm, ensuring that the shape of the arm follows the shape of the fitting with a 0.25mm distance at all times.