Dave Plowman (News) explained :
You can buy slim spanners at a price. But this doesn't need to be as
strong as you'd need for a nut that size on a car, etc. One of the old
pressed steel bike types would probably be strong enough.

If I need something special like that, I look at making what I need
from something else. I keep old spanners and even the cheap pressed
steel spanners. Old ones can be ground down in thickness, then the
width adjusted to the size needed. Pressed steel ones might just need
the width adjusting on the grinder.

I have even cut the business end off sockets, to weld them to a long
flat steel strip, to make a long reach thin socket. I did that to
access a belt tensioner pulley, to fit a new belt. My homemade cost an
hour, versus the £30 special tool for the job. My homemade allowed the
pulley to be lashed back under tension to change the belt, the ready
made didn't. Sometimes homemade is better than the ready made.