Can anyone recommend a simple single sliding door gear suitable for MDF
cabinet doors 12mm thick? The ones I've found online all have hangers which
screw into the top edge of the door, but 12mm is a bit too narrow and
insubstantial for that.

I could use a plastic U-section moulding on the top and bottom of the
cabinet opening with the door simply sliding inside it, but would prefer the
smoother and more reliable action of wheels on a track.

Many thanks.