OK Gustav, you will know that low energy (infra-red 800 - 1000nm) will penetrate MUCH better than high energy (blu-violet 380 - 480nm)
unfortunately silicon (& germanium) photodevices are MUCH more sensitive to infra-red (peak typically 900nm) and have little sensitivity to blue light.
So ideally you'ld like a detector that is uv/blue sensitive. You could use a VEML6070 and pair it with a UV LED.
As a compromise I'd recommend you use a high intensity blue LED (around 460nm) - or even white - and a silicon phototransistor that DOES NOT HAVE a black coating.
Ebay - loads for little money (a few pence). EG
An interesting alternative would be to use an LED as a detector (yes you can)
https://wiki.analog.com/university/courses/electronics/electronics-lab-led-sensor?rev=1551786227