Interestingly enough, I've been using a box almost identical to the ones that Adam shows in his picture, inside my Yak Bob for several years, to stop carrier bags of shopping falling out.
Mine is clear rather than dark green, and the base has started to fall apart, I suspect due to the plastic getting brittle with age, as the plasticisers evaporate. Luckily that doesn't really stop it's use, since all I really need are the sides, since the mesh base of the trailer itself stop stuff disappearing in that direction.
I've wondered whether with a Carry Freedom, you could simply make a set of four wooden boards that clipped onto the edges of the trailer, and each other, with something suitable from a DIY store. That could then be folded entirely flat when not in use.
I may need to invest in a Carry Freedom myself, since I fear getting two cats into one carrier is not going to be an option for much longer, and the Bob won't easily support two cat carriers!