This will hold up to 5 whiteboard pens. It is designed to fit the standard "Expo" brand pens (which are pretty common). This is commonly used for drawing music staff lines on a whiteboard.
There is a cap holder to make the removing of all the caps possible at the same time. This is somewhat difficult though since there is a lot of friction with 5 caps at the same time. The better option is to use the Cap_Replacement.stl which should slide right on to the pens and keep them from drying out.
There are clamps to make the removal and hold of the pens easy. Just wrap an elastic tightly around the body and clamp of each pen insert. There are grooves built for the elastics.
This is made for right handed users with the finger grooves. If you are left handed then just mirror the file in your slicer and it will become left handed!
This is made with a 0.2mm tolerance around the pens/caps, which means that if your printer prints with a larger tolerance around tight fitting parts then you will need to drill or sand out the holes.