depends on the size of the crack
and how far along it runs
as it is a bally backglass, you need to be able to remove the glass as a whole to access the backbox. If you put perspex on the whole backglass as protection mechanism you may make the "glass" too thick and it won't fit back in the backbox channel.
Further, the perspex is likely to rub against the displays. These get hot and could burn/melt the perpex. It could also damage the display glass if there isn't enough room.
best idea. Take a photo and post it - then the gurus here can chime in
