Mounted the sides to the bottom skin. Found it was easiest to do this w/o having the longerons on the top and without attaching any other side channels to the side skins. Once sort of positioned correctly it's just a matter of lifting slightly and making sure the vertical L angles go on the outside of the longerons.
After getting them in place I clecoed in the top longerons and also clecoed in a couple of side channels and the 3 part channel at the top.
Great to have it coming together and looking like something resembling an airplane:)