I have two types in my cellar, one has a rectangular head (with rounded corners, to allow it to be slipped into the slot and then rotated into position), about 10.2 x 6.75 mm and 4.1 mm 'high', and the other has a square head, about 10.6 x 10.6 mm and about 2.8 mm 'high'. The latter is M6. The former also has a ~6 mm thread, but somewhat coarser than standard M6 (but comes with its own nuts). The head of the former (4.1 mm high) would obviously need a little filing!They're also readily available without the attached 'rings'. As has been said, they can usually be found in big garden centres, and also the likes of Homebase and B&Q etc. I'm not sure exactly what there dimensions are (but I think I have some in cellar, and will have a quick measure in a moment!)
I think you would find a number of variations on this theme in the shops.
Kind Regards, John