This is my first instructable and I'm not that good at making stuff, so don't blame me if this doesn't really turn out so well...
This is basically a simple DS game case so you don't need to buy one.
Step 1: Materials
-Elastic Cloth (from a waistband or something)
-DS Cartridge to fit
Step 2: Cutting the Cloth
Mark off a square of cloth the same size of the DS game cartridge. Cut it out and repeat for how many squares will fit in your pen case.
Step 3: Mounting
Mounting the cloth is very simple, just take two thin strips of duct tape and tape it to both sides of the cloth, then affix it to the inside of the case. Slip your games in, and make sure they don't move around a lot under the cloth.
If it does, just make the cloth a little smaller.
If you have any better ideas, please comment.