Steve,
There were several manufacturers of accessories such as boot racks, the most common were SUPLES and EYRAM. I have had examples of both and found nothing to choose between them.
This is the EYREM version from my Légère, now sold ….

And this is the SUPLES model currently on my 15/6 …

The width is important and, although I have heard of Légère racks being modified to suit wider cars and possibly vice-versa, I would strongly recommend getting the correct size in the first place.
Period ads for both are shown on page 13 of the Accessories catalogue which can be accessed via the Technical info pages of the Members only area of this forum! ….
https://traction-owners.co.uk/wp-content/uploads/2017/02/Accessories.pdf
They come up fairly often on French websites such as Le Bon Coin. They are not cheap and then there is the additional expense of getting them to the UK …. assuming you can persuade a French vendor to be bothered to parcel it up and ship internationally, because such items tend to be advertised for collection only and the UK might as well be on the moon for Monsieur Average.
I did collect my last one from the seller but, more often than not, I have purchases sent to a friend in France and then pick them up at leisure.
If the member in question would like to discuss further I shall be happy for him to contact me.
B….