Put a new cover on it plus a warning sign
If someone falls in, they'll have no problem proving that you knew about it and didn't remove all possibility of them doing something to earn a Darwin. A lawyer will have a big payday at your expense. You need to protect yourself as noted above.
Do this right away. Specially since you have occasional campers. Sooner or later someone is going to fall into it and you will be liable for their death or injury since you knew about the hazard on your property and did nothing about it. Protect others and yourself at the same time. A fence is probably a good thing to do in addition to a cover and a sign.
