Back of the house is facing a park, so want to make sure nobody walking
there is tempted...

I have a false alarm cover/box that matches the real one at the front of
the building. I would like to hang it on the wall at the back, but if
possible, add a flashing LED to it. Something that would be powered by,
say, a 9V battery, with a red LED flashing every 20-30 secs (adjustable
if possible), and the battery lasting a long time (a year? two?).


Any suggestion for a turnkey solution? Alternatively I am happy to
assemble something if given a diagram.

TIA.