Twin Point to Point beams, next size up for the length required.
Example, need 50mtr use beam that covers 100mtr, get the idea?
and what ever you use, spend a good length of time lining them up, screwing them on the wall and powering them up to find that they work, phew! is NOT the way.
Then, you could connect them to your DIY alarm.
I've got a pair in a similar situation, NEVER, a false alarm, they do only work early warning buzzers, never false alarmed. They must be 20yrs+ old, used in two properties, still going strong. Make sure all is waterproofed.