Also, 3d printing is great for your prototypes and originals, but depending you your print time, you may consider other duplication processes. Like using Oomoo 25 and Smothcast 300 to make rubber molds and plastic casts of your 3d printed items.. I am happy to share all I know about that!
Example: Use Oomoo 25 to create a silicone rubber mold of your d20 holder. 3 hours total to make the mold.
Use that mold to make 6 copies of your d20 holder out of smoothcast 300 (about 15 minutes each). (Or print 6 of them, however long that takes.)
Make a silicone mold of those 6 copies. (2.5-3 hours, because you're experienced now at making Oomoo molds)
Using Smoothcast 300, you could then turn out 6 d20 holders every 15 minutes or so, while you were in production.
I'm good at this kind of low level fabrication. There are others around who know more about larger scale production.