Quoted from John_I:Nothing holds the protector down. If you make it right it should be able to move a little bit in all four directions before hitting a post or something. This might seem odd. The reason this is done is because the Playfield is wood and when it heats up it expands at a different rate than the plastic does. When this happens the plastic will bubble up. That's the reason the play field protectors that are available commercially work the same way.
Do you worry about dirt and miscellaneous dust getting under the protector?