Unfortunately I just experienced what you all have been talking about. One of my inserts came loose (V2 #89). My quick solution was to put back the stud but secure it from behind with a M3 30 mm screw and a nut at the outside back of the heat sink. It's not elegant and it took some difficult fin grinding with a Dremel, but it works. If the other insert comes out I'll try drilling and tapping for a 3 mm screw. My tapping set is only for SAE--time to get another metric set.
So far the heat sink seems to work. Some of my PA MOSFET failures may have been from overheating but every time I've replaced them the clamp was on tight--it only came loose when tightening the last replacements. I now also add heat conducting grease on both sides of the mica insulator--that may have been the weak spot.
By the way a recent failure occurred just during the PA_BIAS potentiometer adjustment and I know the bias voltage was below 4 volts--and one of the finals shorted after only about 1 second so it couldn't have been from thermal stress. These IRFZ24Ns from Mouser. Perhaps it is a bad batch.
Jack, N6LN
John Terrell, N6LN