Sandbox is a BF2 mod that is made for in-game free-form construction, You can make anything from Battleships to Giant Helicopters, a lot of different materials are available including; Girders, Boxes, Crates, wire fences and a lot more!
You do not need the server files unless you make a lan/internet server. To play it you need BF2 patched to version 1.41.
Well the public beta came out a few days ago, so you probably want to check it out. It had a few bugs but Elxx managed to get everything running alright. There are still a few bugs, but its a beta, so deal with it.
You can get it over here