Copy of PM:
Just had a look at replies. I mentioned that supplier because I already use their little 12w amp.
Arising from comment by the SVOX guy, my design may be prone to feedback and would then definitely need a PTT switch or perhaps that difficulty could be resolved by judicious positioning of the mic & speaker and volume setting.
To attract the attention of my chauffeur I sometimes use one of these:
Reminds me of one of our RAF Chipmunk instructors who carried a short piece of 2"x2" timber for emphasising his 'advice'.