[Piotrsko]

Do you have a 2x4 and a way to put a saw kerf down the 3 1/2 long edge? This is assuming the flange you want doesnt exceed perhaps 2".
Depth of cut would control the flange width. Let it cool back to rigid, then continue bending the flange back on itself with a couple of passes more.
If you need a radius, file or sand an appropriate one on the 2 x4. if you need a hump before the edge, a piece of finger molding up to a 1"round......

Ideally, this also could be done as a vacuum formed thing it you build an appropriate male and have access to a shop vac