clamber onto (something)
To climb onto something in a lumbering or uncoordinated manner. The bus swayed as the boys clambered onto it, whooping and laughing.
clamber onto something
and clamber in (to something); clamber onto climb onto something clumsily. The kids clambered onto the tractor and tried to start it. The wagon stopped and the kids clambered on. All the campers clambered into the bus.