On 17 Jul, 18:44, Geoff wrote:

I doubt they are tailor made as opposed to just being made in a number
of sizes.

Many are tailor made, to some extent. There are a number of plates, so
as to give some articulation, and you can order each plate to be of a
range of standard sizes (i.e. are you apple-shaped or pear-shaped).
Covers are then sewn up to fit the particular set of plates that
you've ordered.