I doubt welding would be very cheap. If you have steel pipes or tube, bolt the ends down from the bottom, so nothing sticks out of the top at all. If its a longer rail you can drill some holes and add some screws or lag bolts in the same way, just drill a pilot hole through the metal thats just big eneough to allow the scew to make it through while still having some holding power. I'd recomend building the rail without the metal and adding that last though, otherwise getting supports on wont be easy.