Consider two first aid boxes .....
Ours is in the cabin but that's often locked. So I've bought a second First Aid box and put it in my Land Rover: no point in trying to lock that. So you might want to have two first aid signs.
I'm about to put up a prominent sign telling everyone where the eye wash is. Yes, a gust of wind the other day blew a load of ash from the site fire (from the old shed) into my face as I walked by. I was wearing safety glasses. But with my recent record of bad luck and bad judgement, I thought it prudent to get some eye wash.
Consider a Keep Out sign
I can't remember where I read it, but if you don't formally tell people to keep out of the site (and fence it off) then anyone might reasonably assume that they have permission to enter.