Yeah, carrying 18ltrs on ones back is doable but, not on a regular basis, else you'll do your back-in. However, it seems like you only want it for the odd jobs here and there, so no problem.
Gardiners do a decent one. I've got there's and it works well, no issues. It comes with back harness which is removable. You are then able to carry the backpack by the lid which has the handle built into it. I don't have a problem with that, but some on here find it awkward. I'm sure they'll let you know
The newer backpack they do has two speed settings which is better than the older version which I have.
Other companies to consider are : Window Cleaning Warehouse.