The top side of where the nut would be located is a trim upholstered cap which runs the width of the opening. It is held down with screws. Remove the trim piece and you should have access to the nuts. Also once your top settles down your J hooks can be to long to snug up the top. You can take a 1/4 to 3/8” off the thread end of the J bolt and make the fastener tighter when you latch.
2010 E350 6.0 4x4 EB 50 SMB
2012 E350 V10 4x4 RB 50 SMB (sold)
2007 E350 6.0 4x4 RB 50 SMB (sold)