Is there a trick or tip to tighten the plug clamp around the cable sheathing to prevent water penetration?
My Leviton 30 amp male plug (as well as Cooper 30 amp female plug) would not clamp down all the way when using the #10 patch cord, leaving small gaps around (not sure the exact term, but it is rated for outdoor use and is flexible - stranded copper in black, red, white and ground wires).
Copper did provide four, small plastic clips but the instruction did not clearly explain what and how they were supposed to be used.
According to their instructions, both plugs are rated to be used up to #8 outdoor cable.
Due to having no power, I reluctantly used duct tape to seal the gaps but would like a correct solution.
You will never get the clamp all the way down with a 10/3 cable. That is how it works. Duct tape is silly and not needed.
Your generator inlet is designed to keep the cable end dry and most decent generator manufacturers have a roof or plate over the receptacles.