When you're talking about attaching a frame out in the airstream, I wonder which sorts of loads you will expose this frame to. Usually "frames", in their classical definition, are intended to transfer their loads via shear-clips to their adjacent skin. If you introduce what could be termed as out-of-plane loads, you will have to deal with an unstable frame and should be address by a method that would account in some way with the concept known as "instability of outstanding flanges". An alternative to such an excercise would be to have two frames with an intercostal tying them and the adjacent skin together to have more of a shear-loading load-transfer scenario (more stable and easier to analyse)