That PC whose fan dropped off the other week has been bodged into working. I used a couple of plastic strips and motherboard mounts to hold them to the board with screws in the right place for the heatsink; but I found it needed more pressure on the fan for it to make good contact with the CPU, especially at the top; so outcomes the wood. This is working reasonably well; although the wedge positioning is a bit touchy to get it right.