The Surefire forends come with one in the box. It's basically just a large, flat piece of metal. There are two dimensions of interest (well, maybe three, read on): thickness where it engages the nut, width so it engages the nut fully but still fits inside the forend, and (third) enough length so you can apply pressure to it. I can give you those dimensions and it would be a snap to make up.
However, I like Gary's idea better. If you're interested, I can email or post the dims for that...I can get ahold of one of the Brownells wrenches.