I would recommend either
a) sending it in for repair somewhere
b) cover the hole with a piece of rubber/silicone and use black gaffer tape to carefully cover it
c) if you feel adventurous, buy a dead K20 and look into replacing parts.
Using any glue to attempt to bond plastic or metal alloy is fine as long as you're not going to put any stress on that piece. In this case, you would never be able to trust that repair to handle a flash bouncing around putting stress on it.
Mind you - glues and epoxies can claim to hold hundreds of pounds of weight and all that, but you can't lather up the camera with goop as there's just no room for that with the popup flash in the way.
Either way, best of luck with the repair -