If you fastened pieces of 2 x 4 lumber in there it would still be an improvement. I've seen guys use single round tubes, double smaller round tubes, 1 x 2 cold rolled rec. tubing , 2 x 2 square and 2 x 3 rec. as well as the laser cut stuff. When you get up to 2 x 2 square, 2 x 3 rec. or the laser cut pieces you'd have a hard time telling the difference unless it were a full-time drag or autocross car - in which cas you'd probably have a bit more tubing inside helping to keep things from twisting.