I have read, but I have not yet made the change, that one of the best locations is the lower transmission fastener on the right-hand side (brake pedal).
This requires a longer cable.
Ken-in-Oklahoma on Boxerworks talks of fastening it to one of the transmission cover screws, with a longer-than-stock screw, and some washers. On the left-top side (I don't think I've seen a picture).