Never seen stainless steel pattress boxes. You don't actually see
the pattress box as the socket covers it!

Well, if mounted to the bottom of the cupboard, then you have to use a
surface mounted pattress, which will be visible. They are widely available,
although frequently with the socket provided rather than separately. They
are particularly common in Victorian schools and light industrial premises.

See Screwfix item 14687.

Christian.