Here's my design for a tool holder that is attached to my Printrbot Simple on the Simple Tower.
This keeps all those little keys contained in one accessible spot instead of hiding in the tool drawer!
The openings are designed to hold a Craftsman 5/16" box wrench, 2.5mm allen key, 1/16" allen key, and 1.5mm allen key, in order. I was able to pick up all 4 tools for less than $10 from all-US sellers on eBay. There are also two small pilot holes in place to be drilled to any mounting size you require.
Enjoy and happy extruding!

• Print out the design (no supports needed)
• Run the allen keys through their respective holes to clean out any stray hairs
• Drill and countersink the mounting holes if required
• Mount it on the side of your 'bot
• Never lose one of those keys again!