Might I recommend the “Ogiue” costume.

Ogiue wears plenty of outfits throughout the series, so clothing-wise there shouldn’t be too much trouble. You can go with the classic hoodie from her first appearance, but make sure to wrap your arm in bandages to go with it. Alternately you can go for Ogiue incognito at Comic Festival. Wearing men-sized shirts is also Ogiue-appropriate. And there’s always the Renko cosplay option.

The hair of course is the difficult part and seeing as my hair is not long I can’t give any real tips. Perhaps you could construct a fude out of a spare large paint brush, or create some sort of fude clip to add to your existing hair. If this is too much trouble, don’t forget that Ogiue has other hairstyles, such as the high school hairstyle (perfect in conjunction with incognito Ogi), the junior high style, or even the heavy side bangs from volume 9.

And to top it off, secretly carry some yaoi.