JWA,
If space and weatherproofing are both issues, have you considered using a reed switch type proximity sensor/switch and a small permanent magnet?
With their contacts sealed in a glass tube (which is often enclosed in a plastic housing that can be filled with RTV silicone or other filler), reed switches are inherently waterproof.
The only drawback to reed switches is current handling. Would your alarm system be passing more than 0.5 Amps through its sensing loop?
If not, here's a possible solution that's small in form factor, and very cost effective. However, you might need to fill the housing around the glass tube of the reed switch with RTV yourself, for vibration resistance, if it isn't already potted with something suitable:
https://www.amazon.com/Magnetic-Norm...s%2C202&sr=8-1
Mike