This is a quick cap design that fits the Corsair H1XXi series of liquid coolers. It should fit all H1XXi's, but it has only been tested on the H150i.
Fits with the original screws that come with the cooler.
Included are two designs - one of an aperture, one of a gear - plus a base STL that you could modify with, say, Blender if you want to create your own design.