Hi,
I just had a look at where CoreFilter actually came from and it was a(n unfortunately failed) [link|https://www.kickstarter.com/projects/1082381665/corefilter-air-filter-base-for-apple-mac-pro|Kickstarter program] from 2014. I literally just learned that I must have been ***extremely lucky*** to have been able to buy one on Amazon ***at the time***. There were only 10 backers to this project, according to Kickstarter, so it's unclear how many were created that ended up in the market. I reckon there's a very slim chance on finding one, but you never know.
I sold mine as part of my Mac Pro at later time, so mine is gone. In hindsight, it really was what Apple should have built in. I haven't looked at 3D printable options as I sold the Mac Pro quite some time ago, so I can't help you there. The same with the original creator: I don't have a connection nor knowledge of his whereabouts. Try social media?