I ended up having to undo the "cam lock" screws to allow the wood to bend enough to squeeze it in. The real problem was with the faceplate, which was ever so slightly wider than the rest of the receiver. The rest of it, I could push in without having to undo any screws, but the faceplate required the screw treatment. Luckily, the cams the screws grip are still in, and I got the cam lock screws turned back a little bit to hold them in place... and the tight fit means the receiver probably wouldn't fall down even if the shelf did.