Hi mate I've done this a few times. It's a bit of ball-ache sometimes, but it's do-able.
I normally have to 'adapt' a couple of boxes, or more, to make something big enough. Use lots of packing tape to tie the parts of your box together, then what I do is go round and round the box with the tape, round the sides in each direction and making sure I cover all the joins in my box pieces. Make sure whatever you're packing can't move about inside the box, as stuff can migrate to the edges otherwise and get damaged that way.
This way I can make a box the right size and shape for just about anything.
Biggest thing I shipped making my own box was a Peavey 1516 cab, she was 48 kilos! I've sent valve amps and all sorts, if you're careful it will arrive in one piece.