I've heard that you should use tubing that is similar in wall thickness, but equal to the ID of the frame to slug it. The solid slug does not allow any sort of flex, but a smaller diameter pipe with the same wall will allow some. I'm no welder, so don't take what I say as correct, but it's something to look into. Also, I have a Menard's close by that sells some tubing. You may also be able to find somone local that has scrap laying around that will work. You probably need less than three feet, right? I have a fab shop that sells the scrap they don't use for 50 cents a pound. You might want to look into something like that.