What you'll need:
Computer with 123D Make (free!!)
Blender (the modeling program, not the smoothy maker. Also free!!) - optional
A 3D model (If you don't know how to make one, you can easily find one for free online)
A 3D printer or printing service. I'll be using Shapeways in my example as I'm most familiar with them.
Filler - Anything that you can fill your model with. I'm thinking colored wax would be awesome - optional
Please note, if you'd like to make an even cheaper prototype, check out my instructable on prototyping on the cheap.
Step 1: Optional Hollowing Step
Right click the cube that pops up when you open Blender. Hit the Delete key and click Delete on the popup that comes up. File>Import your model. Right click your model. At the bottom of the window will be a select box that says Object Mode. Click it and select Edit mode. Hit the A key to deselect all. Now at the bottom of the screen there are 3 boxes that have cubes. The first box is selected with a point on the corner of the cube. Got to box 3 (the one with a face of the cube highlighted) and select it.
Almost there, now zoom into your model and select any face where you are making your opening. Right click the square in the center of the face to select it. If the face is part of a flat surface that you wish to make the opening, hold Ctrl, Alt, Shift, and press F. This will select all linked faces on that plane. Hit the Delete key, this time there will be more options other than just delete. This time click Faces
Switch back to Object Mode, and right click your model again. On the right side are a set of tools. Click the wrench (When you mouse over, it says Object Modifiers). A drop down will appear that says Add Modifier, Click that and select Solidify. A new box will pop up, click inside the Thickness field and set your thickness to the minimum thickness allowed for your material. For my example, we're aiming for Shapeways WSF which has a minimum thickness of .7mm. This is a larger model, so to be on the safe side we'll aim for 1mm. Which to be even safer (and because I use inches) I'll use .04 inches. Note that Blender units are what ever your imported model is. Click Apply, then File>Export your model as an OBJ.