I need to replace a broken window in my metal roll-up garage door. The window casing is surrounded by cheap plastic molding. I was able to get the broken glass out, but I don't see how to get the new glass back into the casing. It seems as if I will need to cut the molding to put the new glass in. Is there an easier/better way?

Also, is there any reason not to put plexiglass in instead of glass? I don't need any insulation factor or anything like that due to the fair weather climate I live in. My kid broke the glass playing ball against the garage door, so plexiglass would be better.

Thanks!