Armrest for Older Chairs

This will fit some old chairs from the 70's roughly at 171MM long and with a 26MM wide slot and a 8MM depth for the square metal tube it mounts too. I'm still working on my main one with a curvature and using TPU to make it. In the meantime I will use PETG for testing since the armrest before had no give whatsoever. I believe in the future I'll use threaded insert to mount these or maybe rely on friction fit. The picture was a quick rough draft that I've been using in the meantime.