I bought an old Dell workstation to make a server. It had an old crusty 92 mm fan in the back. I had some 120x15 mm Noctuas laying around and wanted that sweet silence from Austrian fan gods, but it didn't have space for a 120mm fan, so I decided to make my own converter. It's 10 mm in depth, so with a 15 mm fan it has the same density as a normal 25mm fan and fit perfect in to the case.
Is it pretty? No
Can I see it? No
Does it work? YES!
Hope it can help some other people :D
Info:
Supports: Organic support for the 92 mm holes
Other parts: 4x M4x20 screws for the 120 mm side and 4 M4x8 for the 92 mm side.