The focusing screen of large format cameras is made of glass and is particularly at risk during transport. Cameras that are designed for field use often have a flap with which protects the screen. With the Linhof Technika, the Horseman FA or the Graflex Speed Graphic it is part of the focusing hood, but many other cameras for field use do not have such protection. However, you can make a suitable perspex insert yourself at low cost.
The only component is a perspex plate about 3 mm (3/32 inch) thick in the width of the sheet film cassette. The length is such that it is twice as long as the ground glass frame, plus about 20 mm (3/16 inch). For my Shen-Hao 5×7, the plate size is 148 x 470 mm (5.8 x 18.5 inch). You can have transparent, white or black plates cut to size at the hardware store.
If you still have some leftover perspex, you can also bring it to the right size. To do this, cut a notch several times in the plate with a cutter and an angle plate. Be careful that the knife does not slip and that a straight line is created. The prepared board can be broken over an edge. This requires a relatively large amount of force and some self-confidence, but it is possible up to a board thickness of 3 mm.
You can use a file to deburr the edges and round off the corners a little.
Then push the plate behind the ground glass frame and mark the position where it is to be bent. To bend the sheet, place it up to this position against the edge of a table so that the longer part protrudes over the table. Using a hot air blower and a narrow blower attachment, the Plexiglas is carefully heated along the bending point until it becomes malleable. The plate is then first bent to a right angle.
To prevent the Plexiglas from melting, do not heat the same spot for too long. It is best to move the hot air blower back and forth constantly so that the Plexiglas heats up slowly and begins to deform. Then let it cool down until the Plexiglas has solidified again.
The plate bent at a right angle is pushed behind the ground glass holder again and the necessary thickness is marked. The plate is heated with the hot air blower at this point as before and bent so that it is parallel or slightly converging.
That was it...
I wish you success!