Saw these in several of the shelters in the SNP.
People even asked what they were for in the shelter registers, so I do not feel like it is a completely stupid question....
https://lh3.googleusercontent.com/-9...173%252529.JPG
It is about 3" around, 6" long. Fastened over the opening with bolts backed up with a half a dozen washers so as to isolate /insulate it from the wood....
Also has a slot cut in the front to slidesomething into/....?