I just made one in about 5 minutes with a piece of scrap 1/8" x 3/4" flat aluminum stock by clamping it to a 10mm wrench and cutting down inside the wrench flats with a hacksaw to match the 10mm open-end wrench's inside flats. A bit of filing and a little sanding to touch it up and I've got my own custom Weinmann 10mm brake locknut wrench!

Emergency over, nothing to see here...