Allows a standard office chair castor to be attached to a sheet of 1/2" (12mm) plywood. I'm actually using this to build a moveable, catproof barrier.
Yes, the castor is at an angle in the photo. That's because the castor's shaft is bent (thank you TSA).
Print, check your bolts and bits fit, and screw into place with M4x50 bolts, nuts and washers. Pointy end goes up. Shove shaft on castor into big hole on blunt end.
I've improved the design since I took the photo to have pretty much the same strength and shape but use less plastic.