I use the 32 L (https://www.decathlon.fr/p/sac-a-dos-de-randonnee-multi-poches-32l-escape-500-noir/_/R-p-334334?mc=8649351&c=noir-fume https://www.decathlon.fr/

I use the 32 L (https://www.decathlon.fr/p/sac-a-dos-de-randonnee-multi-poches-32l-escape-500-noir/_/R-p-334334?mc=8649351&c=noir-fume https://www.decathlon.fr/p/sac-a-dos-de-randonnee-multi-poch...) as the "under the seat" luggage and it fits into the metal template.