Two short (6"-8") sections of 2"X4" would do it. Put the fender stay between the two blocks of wood to hold the stay and control the bend (over an edge). One could either kneel/stand on the blocks to keep them together, or tie them together with twine or other cordage.

Just about anything will do that will hold the stay relatively straight, and create an edge to bend over. One could also use a pair of linesman's pliers or channel locks, but this will leave tooling marks.