That three roll concept exhibited in each of those designs is how that's done. It controls the force uniformly on the spans between the rollers so, if the material passed through is uniform thickness the resulting bend between the spans is also uniform. You drive the first and third rolls through gears with an idler gear between to correct the rotation. If you had a set of gears small enough you could make your own but as with so many of these things, by the time you fool around with it or if you have to purchase a set of gears, three at a time, you're probably way ahead to just buy one. A quick search suggests the street price of that unit is about $85.
If you're really wanting to not spend the money and IF the material is thin enough you
might get away with two rollers, one sleeved with rubber hose to act as the forming force on the sheet. If it worked at all you could probably get away with no gears, just driving the rubber sleeved roller, and form as small an ID as the adjustable hard roller allows. The rubber sleeved roller could be almost any size which is helpful if you were to have the rigidity needed over the span, plus the hose thickness. In any rig, the key is the uniform force pinching a span of the material. The rubber roll would flex at some distance straddling the hard roller C/L, serving that purpose. I would guess .005" brass might work and maybe as much as .010". Definitely a trial and error job... if it would work at all.
