We got the first one 10 years ago or so and it still works fine.  They must be working fine because we just bought a second one a couple years ago and they have not changed the design at all.

Brower mineral feeder

 I'm not sure ours is the same brand, but the design is the same.  Every once in a while a cow steps on the lid and it pushes down into the compartment and will allow rainwater in, but the don't damage it, just get some wet mineral.

 It has stake holes if you want to anchor it, but the cows don't push it much.

Dave