Cheap wireless remote siren for van

[Heaveho]

Anyone use and recommend such a thing? There are a variety of options on ebay and Amazon, ranging in price from £18 to £45 or so depending on spec. I want something to protect the back of my Connect, but would ideally like to fit two sensors / sirens operating from one remote, so I can have one in the cab as well as one in the back behind the bulkhead.