you don’t need safety glasses as UVC does not penetrate all the way to the retina, but anything like skin gets a terrible time being sterilized
good UVC LED like Klaran 50mW costs about 20$ in quantity naybe 10$, then 20$ for the quartz glass window, likely they use a plastic aluminum coated reflector
so looking just on LED and optics we hit 200$
If they go cheap with 3-5mW LEDs there is no purpose using 18 of them if 2 Klaran can do the same output
below 300nm you need quartz glass (fused silica) borosilicate will block UVC
Ask Schott how much B2O3 it contains, it has to be a glass that has almost none in it or it would block UV
such glass composition is called quartz not borosilicate