So.. after a few days of use, I decided to strengthen the L connection between the base plate and the rack. The transport carrier is OK, but the base plate is the weakest link: it flexes too much.
I got a steel strip (3x 30 mm), and bent 2 square supports (after cutting). I had to take the carrier off the bike for stripping and painting anyway. The hand carrier already had 4 M10 tapped holes for the axle supports, corresponding holes were drilled in the support brackets to connect . 6 mm holes were drilled in the brackets and base plate for connection with M6 bolts.
When the paint is dry, I want to glue rubber (inner tyre, flooring, insulation) on the parts which come in contact with the cargo. Pics will follow.....