Order the Scout Messenger Bag from Wotancraft
Note: I have been advised by WotanCraft (since this video was posted) that they had underestimated the capacity of their Scout and now claim it is a 9L and not 7L messenger bag.
They have re-measured the bag and list the following dimensions.
Main Compartment: 34 x 24.5 x 11 cm (W x H x D)