Assuming you’re starting in the UK the best box I’ve found that fits within the standard BA dimensions is the ATW35 from www.davpack.co.uk - I wasn’t aware of the larger item flexibility referred to above but unless you’re carrying stuff that’s very light you will run out of weight before you run out of volume. Boxes much larger than this become quite difficult to handle. Make sure you get really good tape too. From the same website I’d go for the 3M cross weave glass filament tape.