As you note, basically all helmets are just styrofoam. If I wanted a lesser nutcase with a "unique" look, as I guess you are, I would de-plastic a regular brain bucket (so I still have all the strap infrastructure), shave down the foam with a boxcutter, and finally wrap it in duct tape. Perhaps colored or even multicolored, in a jaunty pattern. Home Despot et al offer duct tape in colors and even prints, like polka dot (KoM anyone?), zebra stripe, tiger stripe, jaguar spots, hello kitty, etc.

Also, I can't quite tell from your pic, but it might be interesting for you to rig up a special ponytail exit ring, or a topknot channel, or some such.