While I was at the EAA airshow in Oshkosh I looked at several of the portable units and wondered the same thing, just how good is the audible warning in a noisy cockpit, when the pilots are wearing headsets.
As for mounting I was told that you can stick them to the dash, or on one of the side windows and that like my GPS they have a temporary mount that can be stuck on any hard surface then easily removed.
A friend of mine had a similar near miss, and afterward promptly had his airplane painted dayglow yellow with a wide blue stripe. The color was so bright you needed sun glasses to look at his plane, but you could pick him out in the pattern on a hasy day real easy.
Mike
NATCA A80