Q:

How to cut a rectangle out of a STEEL ruler? 

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! 

A:

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. 

A:

Depending on how small a square you want to cut out, you may be able to do it with a rotary tool (e.g.- a Dremmel) with a cut-off wheel. 

A:

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.