How to make a cheap microwave proximity alarm?

Hi all. Some kids broke the steering lock on my moped through the night (probably sat on the seat and kicked the steering) so today I've been wondering about a project I had in mind for a while, for a simple microwave proximity sensor alarm.

I see the microwave module can be bought here for £12:
http://www.ebay.co.uk/itm/Microwave...hash=item25b64c248f:m:meQaxAAQhoUHA7D0lUjoKSg

Then would it be possible to connect this to the horn and battery?

And would a remote be needed to turn the siren off?

Perhaps it would be worth buying a kit, and replacing the sensor?
http://www.ebay.co.uk/itm/Scooter-A...962171?hash=item4aeb854efb:g:GUYAAOSwstxVd-Ob

I live in a mid-terrace house, so something like this (down the line) inside the front or back door could one day also be nice, so burglars don't even get the chance to snoop around without a warning 'pip' sounding...

Or perhaps this could be used instead? A parking sensor:
http://www.ebay.co.uk/itm/4-Parking...3447ffaef674a87f6f2de6&pid=100505&rk=1&rkt=1&

I would suggest an alternate solution. Cover the moped externally with something like a stand alone PIR device connected to an alarm, or a shed alarm type thingie. Putting kit onto the moped itself will be problematic as you will have to mount it somewhere securely, and it will have to be waterproofed, including the connectors. It will also require some sort of master on /off circuit as what's to stop it sounding the horn when it's parked at Asda and people walk past?

Two quotes from the sellers description

It sends out the microwave radio energy into its path and waits for an object to reflect the energy back into it.

Microwave sensors send out ultra high frequency sound waves, and detect the reflections.

which do not inspire confidence ion the seller's ability to provide technical after sales support for the device.

Park the moped on a platform that is supported on flexible mountings and fit vibration / movement sensors to the platform

I've just been somewhere where punji sticks were used frequently and effectively. Is that an option or might the council look dimly on the idea?