Here is a customizable enclosure that includes screw stand-offs and counter-sunk holes in the lid. The parameters for height, width and depth are all external dimensions, so to figure out your internal dimensions you will have to account for the thickness as well.
Tip: Once you have the STL for the size of the enclosure you want, import that into TinkerCAD and use "hole" objects to "cut" openings into the enclosure before you print it. With careful measuring and planning, you can save yourself the trouble of having to do it Post-Printing.
You may have to drill out the screw holes if your print is a little messy. Just be sure to leave enough material for the screw to purchase on.