Magisk boot mount bind script

I'm trying to mount the directory of an app to a folder in storage. I've put a script called custom.sh inside "/data/adb/service.d". The contents of the script are:

#!/system/bin/sh

sleep 30

mount -o bind /data/data/is.xyz.mpv /storage/emulated/0/Test

It just doesn't work. Can anyone point me in the right direction please?