I've seen this done a variety of ways;
1. Using a small spring (like inside a ballpoint pen that retracks), attached to the fan, and the both sides of the HSF clip.
2. Using zip ties;
3. Using a converter which has a 60mm X 60mm base, and fans out to a 120mm X 120mm top where you can mount the fan. I've seen people use cardboard, but I think the thermal properties would be horrible

I've also seen metal, but I'm unsure that the gap between the HSF and the converter would be a good thing...
Using a 120mm x 120mm fan would be ideal, since it would move lots of CFM at a much lower dB. Even moreso if you use a resistor on the + lead, causing it to spin a little slower...