I just posted about this in the Porsche MODs and improvements thread, but I think this might be worth sharing with everybody.
In an attempt to further improve the definite character of the selecter axle in the Porsche gearbox I found a very easy way to limit an axle to 90° positions. I think this video tells the whole story (don't bother the bevel gears if you're not interested in the Porsche gearbox). (Actually I don't really know what the name is of this kind of mechanism. )
Without the bevel gears this is all there is to it:
LXF-file here.