Bob Eager wrote:

Years ago, I did this on a stainless steel sink by drilling a hole in
teh side (the vertical part) in an inconspicuous corner. I then crimped
a tag to the earth cable and bolted it on, using said hole. This was
done inside, so only the bolt head was visible outside.

Another way is to use the type of earth clamps made for connecting to
thin metal sections, such as Eze-earth - shown here as type TZB100:

http://www.tlc-direct.co.uk/Main_Index/Earthing_Index/Earth_Clamps/index.html

or a Screwfix equivalent:
http://www.screwfix.com/app/sfd/cat/pro.jsp?id=18129&ts=12208.

--
Andy