Aeotec ZW112 Door/window sensor $37.95 Prime

These are small and thin although the battery life probably isn’t so great. Haven’t seen it this low that often. It’s usually around $55-$60

I do like the size of these.
If you have the ability to install (drill a 3/4" hole). The Monoprice recessed sensor is a great way to go too. Excellent battery life, Zwave Plus, and affordable $30