bad idea, in order to put or remove the bezel, you wil have to dismantle te cab, in order to slide the bezel

regarding your problem it depends on some other facts we ignore
do you want the plexi to be easily removabe or not ?
1) admit your plexi can be slided behind your panel.
cut the plexi the size you need in width (between both internal side), measure the height you need.
cut the exact width (or .5 or 1 mm smaller)
cut the height + some spare
if you can or want, groove the part wher the bezel is placed on top of the screen, this way tou can slide the bezel inside the groove
drill 2 holes on the bottom corners of the cab, use some spare wood in order to fix the plexi.
2) admit your plexi can't be slided behind the panel.
same as above.
groove the lower marquee box piece of wood, groove the panel, this way you will be able to slide it up the down in order to fix it.