By request, here's an adapter for plugging some sort of ipod into some sort of pip-boy. And here's how to design your own!
Go to http://www.freecadweb.org/wiki/?title=Download and click the appropriate link for your operating system. I like the dev release and have found it totes stable, but if you want to play it safe you can use the Stable Installers. If you're running Windows you'll have to know whether your OS is 64 bit or not, you can get that from Control Panel -> System or right-clicking the background of the "Computer" window and hitting Properties.
If you're replicating a physical thing, measure it with calipers (several pair are printable from this very site if you don't have them lying around); FreeCAD defaults to mm, that seems to be pretty common in the community.
In my case, these were all provided by dajcarrol, so - get your numbers however you get your numbers.
The New File icon
Step 3 - load FreeCAD and start a new file
See the icon above - and make sure you're in the Part Design workbench. Or, if yours starts in the Start workbench:
The New Sketch icon
Step 4 - make a new sketch
Click the icon for a new sketch
The Choose Orientation dialog
the Choose Orientation Dialog pops up. Here's an excellent opportunity to think over how we want to print the finished product - face up? Face down? Sideways? Diagonally?
It doesn't matter much if you get it wrong - you can always reorient it in slic3r. I chose the default, top-down view.
The sketcher toolbar
Step 5 - First Rectangle
Click the rectangle button (above) and click-and-drag a rectangle vaguely around the origin - it doesn't have to be perfect, we'll dial it in soon.
Now click two of the red horizontal endpoints and the vertical green line - they should all turn green
now find the "Center" constraint icon - it looks like a red "> <". This centers the horizontal line relative to the vertical origin axis, which just makes things tidy.
Mine is vertical, yours may be horizontal
Do the same thing with two vertical endpoints and the pink horizontal axis:
Now select one of the vertical lines
and hit the Vertical Constraint icon (pictured, looks like an upper-case serif i)
When the dialog pops up, plug in the measurement. Then select a horizontal and hit the "H" shaped Horizontal Constraint icon, and plug in the measurement.
Fully Constrained is good - you've given it all the measurements.
Step 6 - Padding that rectangle
Click either the Close button from the previous image, or the Close Sketch icon on the same toolbar from which you created the sketch
Then click the yellow block icon next to it that just lit up to pad it:
in the menu, set the height of the pad - in our case, 3mm. We don't need anything fancy this time around.
Step 7 - pocketting a hole in that pad
select a face of the resulting solid - it's hard to tell, but it gets marginally lighter.
Now hit the trusty New Sketch button - FreeCAD will automatically put This sketch on the face you just selected.
Just like last time, make and center a rectangle, plugging in its dimensions. We're lucky it's centered, but as we'll see later, it's no big deal to run it offset from the first rectangle.
Close it and hit the Pocket icon, right next to the Pad icon. To do this properly we should tell it to go through all or to a face (and then carefully select the face on the opposite side) - but telling it to cut out 5mm worth of a 3mm thick surface is good enough.
just might give us trouble were we to later adjust the thickness
Step 8 - adding legs
Click the face of the object again, create a new sketch again, and this time - we'll bring in eXternal Geometry. (You can use the X key to enter this mode, or click the icon, whatever you're comfortable with)
For this I selected all 4 outside edges - you can do it with just the top and bottom too, but that has its own problems with constraining the corners.
Now add a rectangle - Important, hover over the top-left corner until a red dot appears by the cursor (that means FreeCAD recognizes you're trying to make them share a point). Then drag to the right-side purple line, and hover until the Point-On-Surface icon appears (sort of a dot on a 90° arc, pictured) - that means it knows you want it to stop on that line.
Same thing on the bottom, but you may have noticed - now it's bloody impossible to select the edges of those rectangles!
I know! And there's no way to tell FreeCAD to put the purple lines /under/ the new lines, or make them thinner, or dotted, not that I've found...but all is not lost.
Go all the way to the left and scroll down until you see the Elements.
Click through the lines until one you like lights up - the last 4 will be the eXternal Geometry, but the 8 before that will be the ones we just added.
Add vertical constraints as normal; all the lines should turn green for a fully-constrained sketch.
Close out of the sketch as normal and pad it up like we did the first one; I padded 12mm, +3mm for the original surface=15mm total (I think I did anyway, won't it be fun to find out?)
Step 9 - Save your STL
You've been saving the FreeCAD file all along, right? Bah, doesn't matter. Save works the same as anywhere, but Export is what we're after. Make sure you select the pad:
The grayed out ones are earlier editions - you can return to them as a sort of eternal Undo buffer if it ever comes up.
and make sure you set the filetype to STL Mesh:
And you're done! Some Thingiverse denizens prefer STEP files, but Thingiverse itself insists on STL. For best results run it through Slic3r (easy enough to google) to check for missing or impossible geometry; I don't understand how it works, so, good luck!