From what I can tell, it's basically one of these canvas utility belts, just with a custom faux buckle added (possibly a urethane or latex flexible casting, to allow better flexibility during stunts, etc).
Based on the lighting this pic shows the best detail I've seen for it...
There's some more pics of the costume on the Batman Unmasked site. If you scroll towards the bottom of the galleries there's a miscellaneous section with a fan film link in there.