Inspired by Banner / Hulk's creation, I too used Radioactive Man to make Martian Manhunter. I used a Batman cape, the bottom of which I cut. I also needed to make the cape look more appropriate for MM, so I used two pieces from an old Doctor Doom cape and used Aves Fixit to make it all work. The belt "buckle" is a wikkistik curled up, glued on, and painted. The rest of the transformation happened via acetone paint removal and repainting. He's available for purchase-- not a lot of ML customs out there get turned into DC characters, so perhaps this is your chance to get a Martian Manhuntet figure of this scale with this kind of articulation on your shelf!