I've made a bunch of them at this point and you definitely want the .5 which equates to 0.020 inches. I made one with the thicker stuff and it was much harder to work with. Just a piece of advice, do everything with hole punches, sharp chisels, scissors and leather punches. Do not try to cut with a drill. Your mileage may vary though. Obviously a laser cutter is the way there go but I don't have one of those!

https://www.amazon.com/dp/B086X1PYW8/ref=cm_sw_r_cp_apa_fabc_X4FbGb4P7FQ37

You will get ball hang-ups if it is too thick.

Yes, that's right. If you don't have any edges exposed you will not have that problem. However, it will be hard to remove the protector if you ever need to clean underneath.

The reason for the large hole cut outs is not just to make installation easier. Plastic expands faster than wood when it heats. If you have the plastic protector go underneath all of the posts, it will be fastened down to the playfield in many places and will not be able to expand and contract. I tried this once and the plastic protector literally bowed upward in the middle of the playfield after the game warmed up. There was at least a half inch gap if not more between the protector and the playfield. This cause the ball to roll like a roller coaster. Rather than re-cut the plastic, I replaced every bulb in the game (it was a Bally EM) with LEDs to cut down on heat. I also put in powerful fans drawing air into the game which created airflow through the cabinet and out the backbox. That made it so the game never heated up at all inside when it was being played and successfully prevented the protector from rising off the playfield.

So a big part of making a plastic protector is making it so that it is not screwed down. It needs to be able to be slid a couple of mm in each direction or you will have problems. In general, I would also make sure the game is 100% LEDs anyway.