I like J'onn's black outfit better than the speedo, and I waited for Mattel to release an action figure of him in it. Never happened, obviously, and now that action figure line is extinct, so I had to take matters into my own hands. While I was at it, I could fix some things: using a Martian Manhunter as a base was going to be harder anyways, because I would have to shave down the tops of his pirate boots, but using a Jemm figure still gave me the height, but also gave me a head with a slightly more alien shape (more than just the brow ridge, and less than the full-on knife-head of the Mattel releases), and it gave me two open hands (which seems more appropriate for the Justice Leagues main telepath than Mattel's two fists.)
I did have to shave and carve and sand a bit to get the tops of the boots and his cuffs off. I had to trim those weirdly long fingers down and reshape the new ends. I swapped the cape out for one from a 7-inch Batman Arkham City figure that I had cut the scallops off of.
I played around with different mixes of green until I found one I liked. I lightened the blue on his cape. I decided to base the placement of the red lines off of what he looked like in the Young Justice cartoon (possibly the coolest iteration of the DC universe so far.) The one thing I did that kind of works, but I might change later, is that the gold bosses on his cape where it comes over his chest are just brass brads like you use to hold papers together.
In the end, I get to put on my shelf the figure, more or less, that I had been waiting in vain for Mattel to release.