The problem with all those using the "red field/white shape/black symbol" approaches is that they could be construed as legally relevant with regard to the letter of the law.
The hydra logo is definitely on the safe side, as would the hammer version. With regard to the Wolfenstein logo, do not use that one as the current game may not be actively promoted in Germany (possession is not a problem as long as you are at least eighteen years old.
My recommendation would be to increase the level of detachment, so, for example leave out the white field and only use a black symbol on a red field or vice versa.
I appreciate the problem, but the current judicial argument is that it will be liable if the symbol in question "may be mistaken for a Nazi symbol by a non-specialist observer upon inspection, withou further research". This pretty much also covers the rune symbol posted earlier, so I'll take it out.
I'll keep watching this thread, but for the time being, feel free to continue the discussion. By the way, descriptions rather than images are perfectly okay.