IR light does not go as a beam unless you really focus it through some optics. It spreads over. Therefore you have to make sure that when object comes in between the transmitter and Receiver it should not leave big gapes on the sides. or it may atleast cover the transmitter fully around.

Another option you can try to remove IR "U" shape transmiiter/Receiver module, from old floppy disk drives. There are plenty available (5.25") at salvage.

Try your luck.

Rauf