The Markins L-bracket LN800 fits the Nikon D800 without MB-D12
This item was repaced by Markins LN810 - see here
" Get an L-bracket
If you area keen tripod user, an L-bracket can make a massive difference. It allows effortless switching from horizontal to vertical framing, keeping all the weight on top and the lens in the same position. It´s far better than flipping the head 90 degrees, which can seriously unbalance the tripod and compromise stability"
(Article from Advanced Photographer UK)