I worked as a glassier: twice.
Never place a sheet of glass on metal, or cement.
My boss used small rubber blocks in a frame: some people like the wooden ones.
Personally, I like using cigarette Butts. They are much softer, and absorb much more shock.
A bead of silicone is great if the entire inside of the channel is well coated as long as it is kept even.
If you are looking for perfection, apply one coating, and let it dry, apply another coat, and install the glass while it is still wet.
Vibration, and shock is your worst problem.
This is in case you want to do it yourself.
I will be doing a windshield soon myself.
If I get it wrong, and mess it up, I will need to buy another one too!
As for plexiglass, I recommend against it!!!
I'm sure it is against the Law.
A sharp piece of plexiglass can cut you much worst than a piece of glass.
Laminated safety glass is a must for windshields.
Temper glass is used for the rest of the vehicle.
You can temper regular plate glass yourself but it is not that hard.
First you will need to sand all the edges, and corners with a belt sander to round them to make them smooth.
Place some thumb tacks so they support the glass within 1/8" of the outer edges.
Place in the oven, and heat it to 350 F
I think one hour will be sufficient.
Look for dimple marks where the tips of the thumb tacks touch the glass.
This is all I can remember off hand.
You could search out "How To" on the internet. I'm sure they have the best way to temper glass.
I have seen where the Sun has tempered 1/2" plate glass when it has been left out over two years.