Nice drawing. =)

How big is the outside panel? If it is small enough (may even work with a large panel), you could try this:

I just hung planter boxes on our rod iron railing and had to improvise because no planter boxes fit them correctly (they are only about an inch wide, at best). (We can hang them from the outside, but you could still use this on the inside.) We measured out an amount of chain at Home Depot and then drilled holes into our boxes (we drilled a guide hole first- otherwise, our boxes being plastic, they would have split from the force of the drill). The holes were on each end of the planter boxes (L and R sides) and on the back (the part that would be up against the railing). We then took a section of chain and looped it through and around the railing, securing it by opening one of the links in the chain on one end and then tightening it around the link in the opposite end (forming a loop). We repeated on the other side. We used a a U-hook at the bottom and center of the planter to stabilize it (you can tighten these down as much as you want) which we also drilled into the boxes. Because there is a slight tilt, you can put a shim (piece of wood) behind the box before tightening down the U-hook. Then tighten the U-hook around it.

This might sound involved, but it really wasn't... we have six planter boxes and it took about 1/2 hour to do them all and cost about $6 without the wood (the tilt on our boxes is very slight, so we decided to go without, but the cost for shims should be minimal).

I posted all this in a blog; see the link below for pictures of the planter boxes.

Here is a link that might be useful: Planter Boxes

Even though mine hang on the outside I had the same problem..... And my solution is very easy.... I simply placed a coke bottle cap between the rail and the bracket, down low, where they would meet. The weight of the soil keeps it firmly in place..... This solution has worked fine for several years now, even with some ghastly storms....

Here is a link that might be useful: {{gwi:2137}}