If folks want to try it on a real Gameboy without all the wires, the Drag'n'Derp works pretty well. It's a Gameboy cart with a USB port for easy writing: http://derpcart.com/
I was looking at these option initially too, but ended up going for the challenge since, both I read some not-so-good reviews and shipping the thing to germany would probably ended up being at least as expensive as buying the soldering equipment :) Building one of those custom GamePaks takes about ~4 hours once you have all the materials prepared (you really need very, very thin wires!) and putting it together for the first time is one of those finger crossing moments which you'll remember for a really long time :)
Pretty sure you could make your own with a 3d printer and one of those kitchen PCB-etching setups you see people using. There's no magic in there AFAIK.
- https://dl.dropboxusercontent.com/u/2332843/tuff.cartridge.j... - https://www.youtube.com/watch?v=yXNEeld8Lq8