old iron tube stock I could find about the place and made a whole set.  Construction and use is self evident from the pictures.  The tubes should be welded to 6mm iron angle stock, thinner is too flexible.  The hole in the holding piece should be a tangent to the mandrel for the stock in use.  I use 10mm threaded stock for my U Bolts because I can find a lot of this material in biulders' skips.  Hand force applied through a 600mm length of steel pipe does apply enough force to nicely bend this 10mm thread stock.