I live in an old house and I have a bunch of vents like this and they aren't really adjustable, they might have been once but are not anymore. Now that I have a 3D printer I thought I'd set about designing a simple clip to allow for adjusting the vent. Works pretty good!

Printed on a Printrbot Simple Metal
0.4mm Layer Height
2 Perimiters
2 Top/Bottom Layers
15% Honeycomb Infill