MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
I made a similar thing and the holes were very tight from my design, as well. I ended up scaling it up 2% to give it the room needed. This would look great with a slot mount on the bottom, so it could be securely mounted to the top of the Ender 3, right of the stock spool location.