Does anyone know how to cut a hollow rectangle out of a steel ruler? By that I mean cutting a small part out of the standard steel ruler while having the outside part still in tact. So a band saw is out the ballpark. Its for a home project and any help would be greatly appreciated. Thanks!
Drill a round hole, and then square the sides with a file.
You might also get lucky by calling metal working shops in the area. If they have a punch of the right size, it would take only seconds for them to punch the hole for you.
A maker space might have the tools to help you here. Possibly a laser cutter.
For the serious metalworker there are square punches, but for the hobbyist one great tool is the nibbler. This is a handheld punch that removes about 1/8 by 1/4 inch of material per stroke. With practice you can make almost any size and shape hole you want in sheet metal. I know Klein Tools makes one, as one I have one. Others probably do so as well.
You can use a scroll saw to cut it out. A scroll saw is similar to a jigsaw because it has a short, removable blade. These kind of saws are designed for very smooth and fine cuts that other saws aren't capable of.
Drill a small hole in the center of where the hole will be that is big enough to receive a scroll saw blade with a little bit of extra space. Carefully cut out the section, and cut slightly inside of the line, giving enough of a margin for finishing. Use a small to medium sized metal file to smooth out the hole, and bring it to the final dimension.
Rotary tools would not do a great job on this because the diameter of the cutoff wheels are probably larger than the hole you want to make.
if you have access to a well supplied maker space, a water jet cutter would suit you well.