In reply to NaCl:
Do you have a local Decathlon store? I'd try going in and asking them if you don't get a reply online.
If you don't have any success with Decathlon then some webbing and buckles etc and you can make a slightly less elegant chest strap or buy one e.g. https://www.decathlon.co.uk/p/chest-strap-for-backpacks/_/R-p-3330 that one isn't in stock but there's loads similar online (searching for universal rucksack chest strap or rucksack sternum strap will get you more options). These ones don't go on the rails on the sack, you attach them around the straps. A bit more bulky but they don't fall off if attached that way.
I use one on my climbing sack as the supplied strap is really flimsy and I reckoned it would fall off.
Post edited at 22:49