I've just bought an emergency boat horn. Actually, it's the only horn we have at the moment and it has a wealth of fine attributes:-
VERY LOUD
No power required
No moving parts
Breaks down to 2 parts for storage
CHEAP (at £3.99 delivered)
Availability:-
A few days via mail order
Drawbacks:-
The operator needs training in it's use (I find that it helps to limber up a bit and relax before operation!)
Spouse may threaten physical damage if you practice with it in the house
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https://www.ebay.co.uk/itm/181429603950?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649