Whereabouts is the land? It matters because where it is will help determine the end result in number of players. People have to be willing to get there / the land has to be accessible.
As far as what to put on the field? I'd recommend starting with cheaper bunkers by using whatever materials you can have access to. Plywood works, pallets with some material to block up any holes work, and of course sandbags. The type of structures and cover are less important than the effect they provide, especially for what is effectively an "outlaw ball" field. If things go well and it turns into what could be a viable hobby business or even a regular business, then consider investing into more unique or complete structures. When it comes to building cover, just make sure that they are safe and that there's no loose debris, nails, or other nasty and unsafe surprises for players.
You'll want to make sure a chrono is available, and that people are using it. Safety still comes first, so make sure guns are chrono'd and masks stay on.