I needed a present for my little sister's birthday earlier this month, and I was totally out of ideas. All I had was a cute little stuffed monkey, when I hit upon the idea to print a cage to put the monkey in. After printing everything out, I put him in with a little "Help me" sign, and he was sitting on a gift card. It went over great.
It was designed in OpenSCAD, and you should be able to tinker with it pretty well. With the exception of small fudge factors, the dimensions are all in the sizes.scad file. It may not work perfectly, as I got near my deadline I started using more fudge factors and experimentation.
The original design was to print the base and bars in one part (old_base.scad), but the bars were too wobbly so I stopped the print. Instead the whole design works like the roof was designed: the bars have little pegs on them that go in matching holes. The gate has tabs running down both sides that slide into specially slotted bars.
Because of the last minute design change, the roof and base are identical except for the cutout. If you want to mess with this I'd suggest making a function to handle it.
The cage is the biggest thing my Thing-o-Matic has ever made. I designed it to basically max out the build area.
You've got to be careful with this since it's so big. The base is 11x11cm, and it's either 11 or 11.5cm tall (can't remember). This means it won't fit on a Cupcake and won't fit on the ABP on a Thing-o-Matic. Your platform needs to be very level, and make sure to turn off outline (since it would be outside the printable area).
Printing the gate isn't bad (since it's not as big) and the bars are even easier. Both the gate and the bars need support.
It's easy to assemble. You'll need to print two copies of the bars plate. Each one has a special bar with a slot in it, which accepts the tabs on the gate. Everything should fit together pretty well, just use some glue or acetone to bond the roof, base, and bars together.
Since I was under a time constraint, the hole in the roof for the gate isn't quite thick enough. The gate will fit in, but it's going to be very tight. Once you've got things assembled, you'll probably want to sand the opening so the gate will slide in better. The side clearances were designed well enough so I didn't have a problem there.
The way I got everything together was by gluing each bar into the base, then once that was dry I glued the roof on and wiggled all the bars into their holes. I found the easiest way to get the two bars that hold the gate in was to tape them onto the gate and then glue them in together. That kept them aligned while things were drying, just be careful not to glue the gate it's self to the base.