With 123D Circuits.io you can generate a custom enclosure for your PCB.

Step 1: Make a PCB

First you have to design a PCB and add drill holes. At these locations, the tool will generate support points in the enclosure. If you resize your PCB, the enclosure will resize as well.

Step 2: Generating the Enclosure

Go to your project page and click on "generate enclosure" button.

Step 3: Editing Your Enclosure

You will be redirected to the OpenJSCAD.org website which is used to turn the solid geometry in a file format you can use for 3D printing or CNC milling.

OpenJSCAD is a variant of OpenSCAD using the javascript programming language. 123D Circuits.io has auto-generated a template enclosure that you can further customize. These parameters can be easily tuned:

spacing: the distance from the enclosure to the PCB
d1: the height of the top lid of the enclosure
d2: the height of the bottom lid of the enclosure

Try experimenting with the code to edit the enclosure even more. It is very simple to put in other text or remove parts of the enclosure where you for example have a button or connector.