I totally agree with Charles on everything he said. He has obviously done this before. I would add that you can get the proper thickness of material easily and inexpensively from:
They are very customer oriented and will go out of their way to help you.
I would also add that the job is made very much easier if you go to an industrial supply like:
to buy some layout dye like machinists use. This is a blue or purple colored liquid that you paint onto the part. Then you can use a marking knife, dividers, squares etc. to lay out lines on the material. Then when you are filing you will be able to tell when you have filed enough.
When you are drilling a line of holes that will become a slot (chain drilling), you will get far better results if you drill the odd nombered holes first then come back and drill the even numbered holes. If you try to drill a string of holes from beginning to end, at some point the drill will wander and you will have a very very hard time getting the hole where you want it. Skipping every other hole also allows you to overlap the holes and remove more of the material with the drill and less with the file making it faster and easier.