Here's a cheap and easy way to mount an 80mm fan on flat-top heatseat designed for a 60mm fan. Pick up a pack of metal mirror clips at the hardware store -- the ones I found are Bulldog brand, part no. BL30885 (size about 25mm long, 10mm wide, price was CAN$1.69). Conveniently they come in a pack of four. They are 'stepped' shape, designed to mount a mirror flat on the wall. There's a hole at one end. All you need to do is drill another hole the same size and position at the other end. Attach the clips to the heatsink (not too tight), align the 80mm fan and attach it to the clips ", wiggle everything around 'til it's centred and square, then slip your screwdriver down between the blades of the fan and tighten the screws into the heatsink. You're ready to go.