This is basically how all my skins are done, regardless of steel or wood frame.
This way it's freestanding, no need to attach to frame, the weight alone and footprint is plenty to look and feel solid, you can't really tell it's not attached, and the cool thing is if you move or just need full access you can just slide it out away from frame.
So your hinged will mount to this, not the steel.
You can do the same thing as this w/ ply rather than 1x as I do/shown here, pretty much the same but maybe a little cheaper and easier depending on skills.