CR-10 50mm to 40mm Adapter for internal cooling mount

Well I'm a huge fan of Noctua fans and currently they aren't making a 50 mm x 10 mm 12v fan.I had just replaced the internal stock CR10 control unit fan(noisier than the stock heatsink fan) with a quieter fan. When very soon after it failed. It was almost as much as the Noctua 40mm NF-A4x20. And Noctua's specs were better at approx 5.53 CFM and a static pressure of .88in H2O. Not too shabby. I now own 7 of them. So I though "I will just replace it with a 40 mm". Easier said then done. Couldn't f