I just stopped at Q-Post and bought some sturdy boxes. 50cm x 60cm x 50cm for 10 riyal. they have other sizes also.

Shipping for me: DHL was 2200 riyal for a 20kg box. other carriers were closer to 24 riyal/kilo. So I'd avoid DHL if you are price conscious.

I do think it is convenient that the maximum linear dimensions on Qatar Airways is 158cm while the above box is 160 :(