I'm a big fan of JB Weld and also of several of the plastic-specific epoxies.
That being said, given the desired application, this is a case where an epoxy putty (perhaps combined with straight epoxy glue if the putty does not hold) is the perfect solution. I have used epoxy putty to rebuild things like knife handles where a chunk has come out around the top of the blade. It dries to rock hardness and can withstand a lot of pressure. There are even varieties that can be used on things like exhaust pipes and will withstand very high heat.
I would try to rebuild the knob itself, using the putty to fill in the missing pieces making sure that the original bits with the cracked edges are embedded nicely, perhaps just ever so slightly overlapped, by the putty. If necessary you can even make the whole affair solid and drill out the center hole afterward if necessary.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
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