Is that just a common 1:1 transformer like this one
Yes, that's exactly the transformer I used. Sorry I forgot to mention that. That transformer does a pretty good job.
If you have access to the official crab box, then I would advise using it. However, if a crab box is not available, the schematic I posted works quite well.
The first two resistors in the schematic serve to mix the L and R channels if using a stereo signal. The other resistors attenuate the signal levels from line level or headphone level down to mike level. To go from higher-power speaker level, you'd want to use more attenuation before the transformer. How much attenuation would depend on the voltage/power involved.
I built my unit in a common plastic project box. A friend of mine expressed concern about noise pickup in an unshielded box, but I have never heard any noise.