Update: You might want to check out a new version I made instead. It features chamfered edges, a groove that is easier to print, and is fully customisable. You can also add a stem or make a hollow version for adding lights.

Simple darning disc with a groove. There are various diameters: 4cm, 6cm, 8cm, 10cm, 12cm.

With the groove, you can attach an elastic which holds the fabric in place and makes sure it keeps its shape.

You'll want to print this with an adaptive/variable layer height to make the surface as even as possible on top of the disc. However, I didn't sand these to make them completely smooth: the layer ridges actually help to hold the fabric in place.