McIntosh early C20 brass bar faceplate restoration

Hello. 

I am attempting to restore a tarnished brass volume face plate and frame along with an internal cleaning of plexiglass plates as well. 

Problem. The brass frame will not allow for removal of the face plates. They fit quite tight within the channel thus not leaving enough play to slide the plates out. The only way I can see is to cut one end of the frame to slide it out then re braze the frame. But..heat from re brazing worries me with the plexiglass being 3 inches away. 

I am hoping someone has tried this restore and found a better way. 

Thanks