Just for shits and giggles I checked Digikey for IR LEDs and there are 140 examples of small surface-mount (SMD) IR LED's available. These are small enough to solder one of their pads to a piece of 0.020" (or smaller or larger) brass rod, and use magnet wire for the second connection then wrap the magnet wire around the rod. Paint the entire assembly any color you see fit. Those would be very unobtrusive objects which could even be moved closer to the dumper. Just plant them 7mm apart. To make spacing them easier, make a base out of styrene with holes drilled 7mm apart and paint it concrete.
The heat shrink sleeve on the original LEDs likely means there is a resistor there, in series with the LED. You would have to take that resistor and install it in series with the new LEDs.
Here is one of those LEDs:
https://www.digikey.com/en/products/detail/kingbright/APTD2012F3C/9646817 It is only 0.08" x 0.05" (SMD 0805 size). There are smaller and larger ones available but I though 0805 would still be small but large enough to be easily dealt with.
I could also likely find photo-transistors of similar sizes.
