An update.
I ended up finding a cheap caddy for the early Mac Pros on ebay that I planned to mod. It was about half the price of the 3D printing services.
I got it in the mail yesterday, and I did it last night. I was kind of in a hurry, as my wife wanted to watch The X-files which I use my Mac Pro for my Plex server and I had it disconnected to do the work.
The mod didn't take long, and the measurements were pretty close to perfect. An issue I ran into was totally my fault. I was in such a hurry to get it done, I started countersinking the wrong side. I was almost finished too.
Luckily, I had enough metal left to countersink the correct side, it just made the hole slightly bigger than the other side which was perfect.
I had some #6-32 screws around that I had to cut to length, and for spacers, I got .140 x 5/16 x 1/8 nylon washers that were a perfect fit to fill the gap.
There was a bit of scraping noise when inserting the drive in the Mac Pro, probably due to the very slightly raised head of the messed up countersink side, but overall a success.
I now have an extra caddy that I will probably mod, but take my time and get it right.
It would be nice to be able to just purchase one with all the correct screw holes, but modding the caddy was not hard, and if I end up needing more, I will probably just mod them over getting 3D printed ones.