We've just been using 8kg poly boxes for 5kg of pork..... from JB packaging....
They also sell 6kg boxes - but we weren't sure if the internal dimensions would be large enough (all set out in their website)
They also sell dehydrated ice packs (you soak them yourself and then freeze them)...
Just had 5kg of frozen pork in a poly box with about three individual ice pack strips (each quite small), and it was all still frozen after 6 hours in the boot of car....
We're picking up lamb at the end of the week - so will know then how much we can pack in...
let me know if you want us to test it out (our sheep are v small though - what weight are you sellling?)