What tool(s) are best utilized for cutting relatively precise rectangular holes in ABS plastic?
If the material is thin (around 1/16th inch give or take) then a hand held nibbler can be good for small square holes. You can drill a round hole to remove some material and then square it up with the nibbler that makes straight cuts. An added bonus is that the tool works when you only have access to one side of the workpiece.
The tip you see moves up and down as you squeeze the handles and it takes a little rectangular nibble out of the material each time.