Couple of things
what grade material and how thick?
how much flange 1/4-1/2-1" ??
to start with trying to beat a flange 90* over a 1/4" thick motorplate will cause the motorplate to flex, thus when you relax the cowel by removing the fasteners, the motorplate springs back and re-installing the cowel will have the dzus holes out of possision. Always start the lip by just tapping down a little to mark the alu and finnish up with hammer and dolly to acheave 90*.
The material for this should be 3003 H14 x .050 and you anneal by carbon soot and burn off, ther are plently of other methods to do the same thing, this is what I do.
This aluminum has the properties to shrink into itself so any waves in the edges will compress as you mallet into a heal dolly and once edge is flat you can use the shrinker to tune the radious--just be smart and lite about it.
If the material you are using is thicker and you are running a large flange--you have to almost double the effort , also if the radious of the bend is too tight you have to reduce the flange width overall and/or weld in material.
If the material is 5052 H32--- that material is not heat treatable with normal methods and really stiff, so any forming should be done with machines designed to that function
