Crafts / Craftsmen / craftswomen (craftspeople, I suppose)

I was just going to get some of this
https://www.ebay.co.uk/itm/114179157488?
as my estimate is I need 4 10cmx10cm patches so it's <£15.

This is a really good suggestion!

A flap which is the shape and size of 'B' so it fully overlaps, giving a double thickness on the bottom?
Rivet through the double layer to hold in place for sewing and also to act as a foot?