I have two new Stern machines that I wanted to upgrade the power supply fans. On one, I will use Pinmonk's internal fan kit. It's very nice and plug & play and is a simply process to install for most people but requires the power supply to be opened. On the other, I printed (Scott Moschella's 3D model) fan adapter and purchased a great ball-bearing fan on eBay that run almost silent at 12V. This external approach is a bolt-on... requires no disassembly of anything. The adapter cost me $12 to get printed and shipped. The FAN was about $6 each.
The Pinmonk may be the cleanest for technically inclined, the 3D adapter approach may be easier for mere mortals.
The 3D adapter file from Scott Moschella:
ebay.com link: itm