I built one. Used two 6 ft 2x4s screwed together (could have used 4x4, but they didn't have any). I bought some industrial outdoor deck carpeting (you know, the stiff grey kind) and wrapped it around a few times for a bit of padding. Then I just added a couple of 12" 2x4's on each end to get it off the ground and stabilize it (they are perpendicular to the BB. DD really wants it higher and someday maybe I'll get around to it but I'm afraid of the stability.
IT's not pretty but it's functional. Actually, in hindsight I wish I would have made it 8 ft. I did it when she was 5 and not doing much but now she's practicing her cartwheels and while it's long enough, if she steps off toward the end she catches the wood stabilizers. When I finally raise the beam, I may add the extra 2 feet. No room for any more than that.
alison