Maybe try contacting the faucet maker. There is a wrench that fits. It
might not be worth it to purchase for one installation. Could a
friendly plumber loan you one?
I have made my own before, but it's a labor intensive job requiring
donor parts. Those shower drain wrenches are made to crazy sizing
aren't they? You could take the one that is closest on the small side
and make a slit in the wrench to open it up slightly. That should allow
enough torque to tighten the nut even weakened by the slit. (Use a hack
saw or tiny cutoff wheel to make a slit. Then twist a flathead
screwdriver in the slit to open it.)