I got a MintyBoost Kit from Adafruit, and since the tins it is made for are not that widely available in Germany I searched for a different housing. I found a couple of metal cigarette boxes on amazon and decided on this version.
The MintyBoost was soldered quickly.
The only real problem was fitting it in the box. The box is way larger than the normal tin, so for now I just placed the module near the cap of the box. This way I can remove the battery holder to switch the batteries. There is even enough space in the box for a short USB cable.
The MintyBoost works fine with the Motorola Atrix and the Sony Ericsson Xperia Arc by the way.