Delayed systemd slock autolock

I've been playing with this autoslock.service file:

[Unit]
Description=Lock the screen on suspend
+Before=sleep.target

[Service]
User=garrett
Environment=DISPLAY=:0
ExecStart=/usr/local/bin/slock

[Install]
WantedBy=sleep.target

It almost works. The problem is that there's a brief period when the screen wakes up where it's contents are visible before slock is triggered. What do I need to do to fix it? I would think there'd be a way for slock to be invoked immediately prior to sleep instead of after waking up, but I haven't been able to crack it.