Ryan designed the Primo with the same tube spacing for the Z axis as the previous version. This means the mounting plate for the Burly (previous version that is used by the Dremel mounts on Thingiverse) can be mounted on your Primo instead of the Primo mounting plates. In addition to the Burly plate itself, you will need to print the nut traps. Note the Burly mounting plate uses different size nuts and bolts than the Primo for both mounting the plate to the tubes and for connecting mounts to the plate.
When the Primo first came out, mounts for many things did not exist, so using the Burly mounting plate was frequently used. That is, the pathway of using the Burly plate is well tested and will work for your Dremel.
As an alternate solution, I’m willing to work with you to make a Primo mount. Unfortunately, I don’t own a Dremel 300, so it would probably take a few rounds of me sending you designs, and you 3D printing and testing them to get it right. I’d base that design on the Burly Dremel mount of your choice off Thingiverse. If you want to go this route, send me a private message.