the best tool I have found for making small square and rectangular openings is a small triangle needle file, also called a tri square file. A drilled hole to start and then careful and methodical work to square up the corners.
Needle files ( Swiss pattern needle files) are commonly available in 10, 14, 16, and 20 cm lengths ( approximately 4 inch, 5-1/2 inch, 6-1/4 inch, or 7-34 inch respectively from tip to end of handle ). The shorter files will have a smaller profile than the larger.
Also, it is handy to have a safe edge on your file. A safe edge is one without teeth and lets you work right up edges safely will not cutting into some part that you don't wish to. Some files will already have a safe edge but for those that don't, a few minutes spent with a bench grinder and a gentle touch so as not to over heat will result in a file that has a safe edge.
cheers, Graham