I have an extra external Shimano 504wh battery and own a Evoc fr trail e-ride backpack
I've only used it once and found it a bit too heavy with the battery and water in
It was comfortable enough and fits well but I found keeping the front wheel down on steep technical climbs difficult
I have a 2018 Merida e160 with the external battery and I'm thinking of making a bracket to carry the spare battery on the frame behind the headstock some how