How to secure glass to table?

I have an old coffee table, similiar to a chest, and I would like to protect the top with plexiglass or glass. Yet I know that as soon as I put glass on top, I will forget and open the chest, letting the glass slide off. If I get plexi, I heard it scratches really easy. I would prefer plexi, but what do you think? How can I secure both glass or plexi without glueing, screwing, nailing etc.? Thanks!
Where can I get plexi or glass sheets?