I am rebuilding a 1980's road bike made by Zebra Bicycle Co. of Japan. Specifically, I have to relube all the bearings and I have to loosen the BB cup. It's about 36mm diameter and the raised flats are 16 mm across. Attached is the pic. A regular open box wrench won't do it.

Any idea if there is an off-the-shelf tool I can get to get this job done. I guess I will have to fabricate a tool otherwise.

Thanks much for help.