i am not sure how to build it, but my idea would be to take a 20' metal pipe and place a styrofome cylinder inside of it. then place a cloth or something that water won't stick to inside of the styrofoam.then block off both sides of the pipe and start filling the inner most cloth with water. then freeze the whole mofo. Naturally the water will expand inside the pipe, scrunching the stryrofoam, either way just pull out the styrofoam and you have a large ice rail. Then i would build a (3ft tall) x (20' long) x (same diameter as the tube wide) scafold to place the ice rail on top of. at the top of the scafold i would make a half pipe shaped notch to put the rail into. then i would jib the shit.