So, rather than letting them stay in a box, I found appropriate bindings for them (from my scrap bag of binding, obviously!), and sewed them on.
It was the work of only half an evening to hand stitch the binding to the back and complete them.
I tried two methods.
This was the way recommended by the tutor, using bag stiffener - a bit firmer than regular felt, but still pretty easy to stitch through - and an outer and a lining fabric.
You make neat little parcels, sewing the raw edges down around each of the six panels....I'm very happy that they nest easily, and think that the 1.5" box, handmade all the way, gives the nicest finish.
It's easy to use them when I have four or five or a same or similar fabric, making little 'kits' to sew as a leaders and enders projec, but sometimes I end up with a bunch of random squares that hang around. Sometimes they hang around long enough to attract friends - more of the same or similar fabrics - but not always!
I saw cards similar to these on Instagram (sorry, I can't remember who posted them - if you recognise the original maker please let me know so that I can acknowledge them) and decided it would be the perfect quick project for me. Now I have several greetings cards that can be customised for birthdays or any other occasion. Perfect!
Bonus! As well as wrapping gifts for my friend, it also helped with the tidying that I needed to to (you know, when your study / sewing room is also your spare room, so surfaces have to be actually cleared for a while!).