Stainless Steel seat post binder Quick Release lever
Works on most Riv seat binder lugs but you may have to use a couple 3mm - 5mm thick washers with a 6mm hole on either side of the lug.
The rubber lever cover is comfy but too soft and will eventually tear with lots of use. Wrap the lever with twine or cotton Newbaum's tape instead.
dimensions of bolt: 6mm x 55mm