Don't forget to check that the flat pieces of glass, which are suggested, can replace the usual neg carrier without getting crunched or cracked by whatever mechanical arrangement positions and locates the standard carrier.
If there are grooves or castings to line-up with grooves in the normal (often alloy or steel) carrier then they would need to be safe for bearing down on the glass too. Some enlargers use the weight of the lamphouse to hold the neg-carrier securely and might crack the glass if the pressure is not evenly supported.
Yes! Careful! My Beseler's stage is flat and only held shut only by spring pressure. My Omega's stage is held shut by the weight of the lamphouse and condenser. It's heavy! If I let the stage close •S•L•O•W•L•Y• it's all right. If I let the springs snap or the condenser come crashing down it WILL shatter the glass.