Regardless of what you use. If you hit it hard enough it will be broken. Conversely if you make it thick enough it will stop or absorb most impacts.

I saw on the show, "How Its Made" they made impact resistant windows. Primarily to stop flying debris during storms. It consisted of 2 layers of tempered glass with a thin layer of a plastic film sandwiched between. The film looked about as thick as Saran Wrap. The whole thing was baked to seal the pane. To test it they fired a 2x4 at about 160mph to test it. The window was damaged but the 2x4 bounced.

The film is the secret. It flexes and distributes the force over the entire area rather than just the point of impact.

To use Plexiglas you would need to build up several layer like laminate armor. You would need some type of flexible plastic sheeting between each layer. They you would need to heat the entire thing somehow. Hot enough and long enough to fuse it together but not so long and hot that it is damaged. Very tricky. To stop what you desire you would need to experiment to find out how thick it would need to be.

To stop hunting lever rifle round I would think you would need to be at least an inch thick.