I shall go and measure the aperture this weekend.
In the meantime I think correspondents would do well to bear in mind what the editor says about terms and conditions. There is a rectangle 'carved' on the south side of stone 8 with clearly delineated corners, and this is ten inches by twelve.
There are others. I am surprised at the lack of comments regarding the 'figurative carvings' on the interior of stone 6. What are they?