This gridfinity fountain pen holder was designed to accommodate the majority of my fountain pens in a nib-down configuration. Specifically, it accommodates a <14mm diameter, <4mm clip depth, and <9mm clip width pen. Do note that smaller diameter pens will allow for larger depth clips (A Platinum Preppy, at 12.75mm diameter and 5mm clip depth will fit fine)
Editing these values should be fairly simple to do in fusion 360; just edit the values in the sketch and it SHOULD work fine. Probably.
From my small collection, this works will with the Jinhao 599 (Lamy safari clone), the Pilot Explorer, the regular Sharpie marker, and the Platinum Preppy.
Smaller diameter pens and pencils, as shown in the pictures, don't seem to work as well.
Larger pens, such as the Noodler's Ahab, cannot be placed nib-down.