I had a go with the zipties yesterday and I've run into some problems and the fan still moves up and down, so is likely to end up resting on the RAM which I don't want.
One problem is that the zipties that I've got that fit through the fan corner holes are too short to go between the fins (as I need to attach another one to the end), so I've had to go under the fins instead. I imagine if I had longer zipties and could go between the fins that would keep the bottom of the fan up enough and off the RAM. Putting them under allowed me to fit the second ziptie to the end of the first one before feeding it under and between the heatpipes but obviously I can't do this if going between the fins as the ziptie head won't fit, so I'd need zipties long enough to allow me to fit the second one after. I've realised I don't need the zipties to go through the furthest hole on the fan, only the nearest as I've done with the top holes but even then they're not going to be long enough. I've got some much longer ones which are too thick to fit through the fan holes but perhaps they make some longer ones that aren't too thick.
The other problem is that the top fan holes are somewhat above the heatsink, meaning the ziptie has to go up at an angle into them and therefore there's a lot of give in them to move down. I'm thinking perhaps if I can get a suitably sized strip/block of firm rubber to put under the zipties that will prevent that but I'm not sure where to get that from.
I probably only need to solve one or the other of these problems and hopefully that will be enough to keep the fan securely in place.