In reply to George Wright:
A wild card......
There are slingshot rucksacs designed for photography. These can be both small ones or quite large in relative terms if your looking for a larger sac, and generally have strap over the right shoulder. At least some have removable interiors, so this maybe worth considering. All the larger photo sacs are double strapped though - that said some are designed to slip a shoulder strap off and still be balanced (to allow spinning sac round to gain access to camera etc.) and having larger waist belts the best designed ones could be quite easily worn with only one shoulder strap on.
Something like
https://www.lowepro.co.uk/brands/lowepro/slingshot/slingshot-edge-250-aw/pd... I have no recommendations, sorry, as I personally don't like the sling shot ones so never researched them.
A word of warning, you do have to pay a premium for the photography ones and some can be 3 or more times more expensive than you would expect for a walking one. Partly this might be for the design and extra padding that is need for protection of camera equipment, and partly a premium for a specialist market.