Ellis's method definitely work well but as with Peter I don't have a blowtorch so I just bore my wheels out to 12.7mm (an odd size I know, it's the diameter of my countersink bit which seems to be a perfect size), place a UNF nut on a 100W soldering iron for a few minutes then point the iron downwards - holding on to the nut with a pair of long-nosed pliers - and let the nut sink into the wheel using just the weight of the iron.

It can be trickier this way to get it concentric and wobble-free though so once it's semi-cool I screw it on to a drill shaft and check for balance and make any adjustments by eye. Then once it's set I use the flat face of the soldering iron to go around the boss and melt the plastic into the edges of the nut just to make it as strong as possible. Leaves your soldering iron a bit messy but it's a separate one I use purely for wheel inserts.

This is the same method I use on the Scot-Bots kit wheels (Banebots) and we haven't had any failures yet, touch wood!