Ideas? On how to get smaller fans to stay in place on Noctua D15

[Helen Barber]

I bought a Noctua D15 black version

The problem is my ram is to high and the other side is to high as well, so stock fans are a inch above the actual cooler.

I would like to place some smaller 120mm fans directly on the cooler, but struggling to work out how they could possibly stay in place.

Any ideas?.... Plastic cable ties?

Thanks