See if you can get it work, Take a small speaker and a glue filled straw possibly with a felt or cork end. Use a 1.5 V battery to get the distance right. See if you can activate the speaker to tap on the screen.
Balsa wood is yet another. It's light and easy to trim. Agin, put some felt where it touches the screen.
Now you have an actuator. You can use suction cups to mount it.
One that works, all we have to do is rig a time that will pulse it every so often.
Make sure the taps will work. Capacitive touch screens might be harder.