What fittings to use for floating box shelves
Unfortunately the MDF board is too thin for true "invisible shelves" I really hate those fittings, they just seem so un-secure!

You cannot raise the connector, since the board is too thing. but what you could do, is run a beading across the bottom edge.  Thats about all I could think of. 

that would also help with the secure-ness of the fitting since the screw heads would not fall out the grapple holders.