Security of Glass doors

[Peter Morrissey]

I purchased a multi-family which has nice old wood doors, filled with panes of glass which could easily be knocked out to reach in and turn the lock. It is an OK neighborhood, but want the tenants to feel secure.

For one unit there is a nice grate on the back of the door which does the trick. It is screwed in though and would be a pain (haha) to clean the windows. No idea where to get something like that for the second door and open to other suggestions. I could screw plexiglass on to the back, but that will make the windows impossible to clean. Maybe if I seal it that won't be an issue, but plexiglass doesn't hold up to cleaning either. 

Looking of course for easy, cheap, and effective if there is such a combo. : )

Peter