I use a product called "J-B Weld". It comes in 5 min.(quick) and regular. This material, when fully cured, 24 hrs., can be drilled, tapped and worked to shape. Like Rodney said, put the pieces back together, carefully, use very little to hold things together, then when the J-B sets up, appx. 5 min. and you are satisfied, fill all around the repaired area with plenty J-B and let it fully cure.
Another approach is to pile up a lot of J-B, 5 min., and let get real tacky, then carefully push the knob onto the cleaned shaft. You must push straight on with no rotational motion and hold the knob till the J-B sets up, appx. 5 min., then let it cure for at least 24 hrs.
**Scuff the areas with fine sandpaper and get the plastic clean for best adhesion**
Soman2
The other Rodney