Custom /usr/bin/xflock4 to use only slock and check for user activity before actually locking the display

xflock4

#!/bin/sh

PATH=/bin:/usr/bin
export PATH

sleep 1
xset dpms force off
sleep 20

# Check if the monitor is On after these 20 seconds,
# which means that the user acted to cancel locking

(xset q | grep 'Monitor is On' >/dev/null 2>&1) && exit

set -- slock
if command -v -- $1 >/dev/null 2>&1; then
    slock >/dev/null 2>&1 &
    # turn off display backlight:
    xset dpms force off
    exit
fi

# else access locking failed
exit 1