I’ve never broken a ground glass, but I’ve probably been pressing my luck.
Pretty basic. Just two pieces of 1/16″ Delrin attached to a scrap of mahogany with 8/32 screws and threaded inserts. The inner piece of Delrin could be a lot shorter, it only needs to be in range of the ground glass frame’s spring tension to hold it in place. Extra holes were drilled to improve the grip when pulling the shield out of the back. I planed a 5° taper to the outside mahogany face for a little spring action to help hold the outer piece of Delrin against to the camera back, otherwise it flapped annoyingly. The Delrin is nice because it’s so low friction, it slides in and out with damaging the wood finish or neoprene light trap. It’s also relatively rigid for its thickness. ABS might be a better choice for this, the Delrin is is really glossy and scratch-prone, but I dulled the gloss down with some synthetic steel wool, which helps to hide scratches. Before assembly the wood was sprayed with lacquer, then I used some double stick tape to attach a strip of velvet where the handle butts up to the ground glass frame.