Update: Added 2 slot and 4 slot OPTIONAL extenders so you can make these as tall as you dare. :)

I decided to remix the Pen holder for 13.4mm and 8 Sharpie Pens (instead of 16). This should also fit the thin expo white board markers. Printed fine on my MP Prusa i3.

Suggested Print process:

Print the "DNA_Helix_Sharpie_Pen_Holder_13mm_BothParts02.stl" STL file
After print, remove two parts from print bed.
a. bottom of helix is outer print
b. top of helix is inner print.
Assemble with glue, or your favorite adhesive to make the bond permanent. (I used super glue).
Insert Pens and display.

Optional Extender Instructions: (For those with more pens and/or just want to go taller)

1. Print the original file "DNA_Helix_Sharpie_Pen_Holder_13mm_BothParts02"
2. Pick either the 2 or 4 slot extender design and print a pair for each level you want to add.
3. The 4 slot extender is going to be a challenging print. I suggest hairspray or glue stick or whatever trick you use to stick a design that tends to lean. If you are having problems with the taller 4 slot design, switch to using 2 sets of the 2 slot design (easier to print).
4. After printing these, they should fit between the top and bottom parts of the original print design. Use super or krazy glue to make permanent.
5. Another benefit of the extenders is that you can make the design more colorful by printing in different filament shades.

I also have a pencil version here: http://www.thingiverse.com/thing:1803253

*Warning: My other DNA structures are properly rotating to the right in accordance with this universe. This model is rotating to the left (huh! oh the nerve of this guy!). You should be able to flip this in the slicer if this is a problem for you.