For example: You could put the LED inside an opaque box with a hole at one end, you could make it more of a "pinpoint" type of light. You could put the LED and detector next to each other with a wall between them.

IRRepeater

Putting a reflector on the other side is the obvious and trivial answer. But it won't work completely reliably as a beam break detector because of reflections from the very objects which are breaking the beam. For exactly this reason a distance measuring is SPAD LiDar module is a better bet. Since your objective is a doorway of less than 2 mtrs, this is perfect. Simply do fast, continous distance measurements. If the result shows less than (say) 4ft, then count it as a beam break. There some excellent cheap modules available now... $5-10 range. I'll post the part number as soon as I remember it.

Bestir reflectors

Source: https://www.digikey.com/product-detail/en/sparkfun-electronics/ROB-09453/1568-1272-ND/5762422?WT.srch=1&gclid=EAIaIQobChMImKyvqPLI4wIVhcpkCh2mpwiNEAQYBSABEgJe5fD_BwE

IRzoom lens

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Look for line or proximity IR sensors. Some even detect distance. Many are compatible with bread boards or can be wired to them with 0.1" jumper wire or headers. They have the detector and sensor built into the same module as the one shown below.

Infrared mirror

You can also use polished aluminum as the reflector. For a recent project I used 3" x 3" x 1/16" aluminum. I sanded it smooth with ultra-fine sandpaper. Then, I polished it with car polishing compound until I could see my reflection clearly.

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Remote Control reflector

A corner reflector has the advantage that it doesn't need to be aligned perfectly, it will reflect the light back where it came from. The retroreflector that peufeu referenced is composed of many tiny corner reflectors. These should work with near infrared, but it would be safest to find one that specifically says it is good for near IR.

The reflected IR beam will work just fine and the range can be quiet good, just experiment with your TV remote and can actually bounce IR off an opposite wall sometimes. I have made many of these IR devices and they can be tricky to say the least. For pin point accuracy place the TSOP at the end of a tube or drilled hole with the inside of the tube or hole being Flat Black. Just bounce the IR beam off of a piece of reflective material, even a piece of aluminum Duct tape. The secret to getting range is how much current you pump through the LED. A TV remote with 3v of power has like a 2 ohm resistor and the IR current is quiet high, but being modulated looks like the LED can handle it. The most I have used for continuous on IR is 200 ohms at 5v, gave me the range I was looking for . You may be able to go way higher because the LED is modulated and possibly will survive for a very long time. Place the IR LED right along side the TSOP, because the TSOP is at the end of a dark tunnel it will not see the IR beam until it is reflected back into the dark tube the TSOP is in. With some experimenting you can achieve what you are looking for. Hope that helps a bit...

Iroptics

I have a working set-up to detect beam breaks using an IR LED, modulated at 36kHz, and a TSOP receiver connected to a comparator. It outputs the count via Arduino. The range is about the width of a door, say two meters maximum.

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.

I am guessing something will need to be done in order to maintain a straight or angled beam and not have it reflected back all over the place.

You can mount your transmitter and receiver next to each other, but you'll need to block the direct light path. Here are two examples of pre-build products. I would model a design after the first example; I'm not sure the isolation of the second example is sufficient.

You need a retroreflector, which is a special type of reflector that sends light back towards its source (unlike a simple mirror).

IRPrism

Is there a way to reflect the IR LED off a reflector and having the TSOP receiver on the same breadboard as the LED? Perhaps behind it? The German term 'lichtschranke' seems to be what I vaguely have in mind, but I don't speak German and can't find it in English.

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.

Infrared reflector mirror

The only drawback was the mirror was super sensitive and the slightest movement would throw the alignment off, even if only temporarily. Mine was mounted on another 2x4 though so I’d imagine mounting it on something more stable might help!

And, yes, you can use a reflector, but make sure it is appropriate to your IR wavelength. For example, a standard bathroom mirror won't work well. These mirrors are often aluminum under glass. The aluminum reflects well, but the glass absorbs IR.

The copper pipe was pretty crucial, the focused beam was detected at least 3x further than the LED by itself. It also helped during setup because I was able to shine a flashlight through back to perfectly align the reflection onto the receiver.

The best IR reflectors are often metal (copper, aluminum, silver, etc). Here is a chart showing the reflectivity of different metals at different wavelengths. It looks like copper, silver, or gold will outperform aluminum at typical near-IR frequencies.