This diagram might help...
wetplate back diagram
It looks like a half bullnose slat of timber is used, set against something springy, that locates into a partial groove made on the far side of the darkslide. It seems like some of the downward slope of the bullnose is left in the channel so that the darkslide can push it aside when reinserted.
I was thinking of building a back using this method or with a circular dowel locating in a partial groove, but haven't got round to it yet. I was also thinking of attaching something to the bottom of the darkslide once it is in the channel to stop it from being withdrawn completely and which would also help block extraneous light.
All the best,