Three solutions come to mind.
First, Crutch tips, Yes the little rubber things that you can
pick up at any drug store or medical supply
house, they go on the end of canes to provide traction/protection. They are the perfect size to slide over an exposed male end of a camlock, they also fit neatly into the recess of a female end. When working on Men Of Honor, I bought out several drug stores entire indventory to cap off the spare plugs on spider boxes, lunch boxes
etc. This solution is going to set you back around ten dollars.
Second, Much more expensive, buy some male cam locks, fill the back end < where the cable goes > with
epoxy, then use the as plugs, this will provide a water tight / corrosion tight seal. As mentioned, however, this is a much more expensive solution at a couple hundred dollars.
Third, Install a new panel with
NEC approved Wet condition,
safety caps on the plugs. This shouldn't be more than a couple thousand dollars.