I have a big plastic tank roller I bought at an auction on the cheap but an old friend gave me one that is 30" wide and about 14" across the end of the roller that was built from air conditioner galvanized duct pipe. They took two end caps, drilled holes in the centers for a piece of 3/4 inch pump pipe to stick through extending about 5 inches beyond the ends of the roller. Stood it up on one end with 1 cap installed, stood the pipe up in the center but had it blocked up off the ground the proper height, started filling the duct pipe with concrete and took a rubber hammer to beat on the sides to move the rocks away from the outside somewhat, just like when forming with wood, filled it full and installed the other end cap. He says it's 30 yrs old and is still holding up well. If the sheet metal rusts away the concrete will still work without the outer skin.
Duct pipe it somewhat cheap, easy to work with and is basically to hold the concrete in place til it dries but after 30 yrs mine is still in good shape with the duct pipe still attached. After it dries, build up a towing frame...cheap n easy build.