I made it at TechShop. http://www.techshop.ws.
I used Autodesk Inventor, which is avail to members, to design the replacement knob/lock. After taking careful measurements of the original knob & slot it fits into, I input them into Inventor. The difference was that instead of drawing the knob on top, I extended the tab on the bottom. That way you can only unlock it from the bottom.