JB Weld is good stuff. thing is though, it cant do everything. if you have a hole 2inches by 3 inches.......it wont work for that unless you have the piece of metal that came out of there. it's great for patching small stuff, but it wont make a large, flat patch like that. if you had a corner busted offa something or a crack to stuff it in....or something like the oil drain plug area cracked and busted, THAT'S what it's great for. it needs to be able to have large surface areas to cling to and relatively thick layer too. spread it too thin and only holding at the edges...and you are asking for trouble.
your best bet for what you have described is to find a piece of thin aluminum plate and have it heli-arced (welded) over the hole.