Finally got around to printing the tweaked design myself with South London Makerspace's Ultimaker 2, with some Filamentive Recylced PLA in glorious Makerspace red (well, some kind of red anyway!)

Here’s the Ultimaker, making away, at 25% infill.

Couple hours later, out comes a finished part!

Although it’s not obvious, the top (which was face down) flared out a bit. Barely perceptible.

Looks good! And once I got it home, it’s only gone and worked perfectly!

Works a charm, and the smooth finish of this filament means the arm swings from side to side with no effort at all.  The lip over the edge stops it from rattling as well.  

Success!