On 20/04/2015 11:41, Tim Watts wrote:

I would say: flush mount a reed switch in the door surround next or just
under the flap. Glue a small but powerful magnet to the flap.

Just buy an alarm door contact or even one of those cheap battery
powered window alarms they had in poundland, self adhesive, contain a
reed switch and a sounder. I think they were a pound for four, I bought
some because I wanted the batteries for something else.