Upload & Sell: On
The Hejnar option looks like a pretty bombproof solution. That's what I would proably buy, if I was doing it today. Of course, the item in the link is only for the plate that replaces the RC4 plate, and it doesn't include the A-S clamp that you'll also need.
The Manfrotto L-bracket looks like it's for the Manfrotto RC2 clamp system, which is a step in the wrong direction. OTOH, for RC2 users, it would be a big step ahead of the 341 Junior Elbow bracket. I converted my camera and lens support to Arca-Swiss clamps and plates years ago, and now use the RC2 and RC4 systems for lighting and etc.