I use an Evoc bag…still not cheap I suppose. I take the bars off, mech and rotors, then fold lengths of pipe lagging over the frame. The sides don’t seem that solid but everything’s always been unscathed. Its been on a few trips and it’s still like new. The wheels are a godsend too, especially if you’ve been used to carrying/dragging a box about.
Hard boxes are too heavy imho, by the time I’ve put spare tyres, tools and shoes etc. my bag is usually approaching 30kg…the limit is 32kg, I wouldn’t be able to get a hard box under that weight, that’s if it even had the space for the extra stuff!