I have a bright screen with a grid and a 45° split focus that I really like. If it fits into an RZ focus screen frame, and it's as easy to change the screen in an RZ frame as it is with an RB frame, I'd rather just do that than spend $50 for another screen (assuming I could find one like the one I have).
OK - the bright screen I snagged for my RZ67ii is a grid. I dug out my RB67 and RZ67 screens for comparison. Here's the tale of the tape:
RB67 frame: 86mm long side x 70mm narrow side x 7mm thick
RB67 glass: 77mm long side x 67mm narrow side x 2mm thick (thick estimated)
RZ67 frame: 75mm long side x 70mm narrow side x 4mm thick
RZ67 glass: 73mm long side x 68mm narrow side x 3mm thick (all estimated as the frame is sealed on the glass).
It does not look like it will be as easy on the RZ screens as on the RB screens. It looks there are small snaps to bend out of the way on the RZ frame. However, it looks like you may need to do some trimming of the edges on the RB67 focus glass to make it fit the RZ67 frame.
Why not sell the RB67 screen and use the proceeds to get the RZ67 screen?
Sounds like a lot of work for me when I'd rather be taking pix or making prints. Whatever you decide to do, best of luck!
Hope my measurements have helped you...