Simple trip line motion sensor

Hey all,
I'm looking to create a simple 'invisible' trip line motion sensor for a project. I'm just trying to stay open minded and collect some ideas/advice on what sensors to use.

I'm going between a PIR sensor and an ultrasonic range finder. My thoughts are that a PIR sensor might fan out too much for what I need so i'm kinda leaning towards a range finder, but I wanna hear some other thoughts or perhaps ideas that haven't crossed my mind.

I don't have a lot of time to prototype because of deatlines so I'm really hoping to just find the best or a really solid option and move forward with that.

Thanks!

If the PIR sensor is too broad, you can (a) remove the fresnel 'bubble' lens covering it and/or fashion a tube/funnel for it to look through with a length and diameter than gives you the sort of 'beam' control you want. That may be of limited success as a really narrow beam may defeat the sensors ability to detect motion [just my speculation].