I would it fits close but there is a small gap
I used it I conjunction with the box but mounted to the structure of the home for a more sturdy installation.
You are correct, flat side to flat side it was only about 6".
yes we used it on siding.
Did not paint
installation. 4" siding
Please contact your local Home Depot and request that they order the item you would like.
The best way to ensure this item will work is to measure the height of the section of siding where the box will be placed to make sure it will fit.