I never found a strong enough insert so I bought a
Lowepro Slingshot 100 from the classifieds for 20 quid. Butchered it with some strong scissors and a Stanley knife cutting off the straps and top section, just leaving the bottom camera storage part.
Left the small front pocket attached for bits and bobs and kept the fold away waterproof cover for added weather protection when inside the rucksack
Works a treat as an insert in a normal rucksack and better constructed than the inserts I have seen.
Goes longways in an old
Camelbak HAWG or width ways in a larger rucksack, just the job