I've had an ongoing issue for a couple of years whereby the knob on top of the selector stick periodically comes off. I've stuck it back on with the likes of metal glue, but it's detached again. So I've bought the hill part:


and it's now arrived, but there's no knob with the selector. It looks like the knob screws on from the underside of the selector as there is a void running up the stem.

Any ideas how to fix this problem for good? Cheers