Practical issue aiming non-visible infrared light at a reflector
Pulse the LED to let you run them brighter without overheating so aim doesn't matter as much.
Build a mounting block. so they are held rigidly in place relative to each other. This may require the ability and tools to drill accurate holes at small angles. It goes without saying to not rely on breadboards when alignment matters.
Consider placing a visible LED with the same FOV above the existing photodiode and IR LED so that you can aim them better. Of course, this would require that those be rigidly attached to the IR devices so they remain in alignment.