you can buy an empty box at the Qpost at the airport road... the biggest size is i think for 50 kilos capacity that cost only 12 to 15 riyals... you can also visit qpost office inside lulu hypermarket...infront of uae exchange..

or if you want much bigger size you can go to filipino souq and look for conor or coriner near gulf horizon hotel..you can buy different sizes of empty box there...