Reflect IR beam off reflector instead of emitting straight to TSOP receiver
You need a retroreflector, which is a special type of reflector that sends light back towards its source (unlike a simple mirror). 

This is what you'd put on the rear of your bicycle to reflect the headlights of incoming drivers back at them so they see you at night. You can get them for cheap in any bicycle store or supermarket. Even better, shown below is one that is designed to be screwed at the back of a trailer. If you want to mount it with a screw, that would be a nice choice, as it already has a hole.



Next, put your IR LED and TSOP receiver close to each other, both aiming at the reflector, and perhaps a black plastic or cardboard separation between the LED and TSOP... Adjust LED power down to make sure the signal is detected with the reflector, but not with IR light bouncing off the people you want to detect, and you're all set.