score:3
You can bring other types of sports equipment and other items instead of a suitcase or instead of an extra suitcase bought in advance, if:
Max. 158 cm / 62 in (l + w + h)
Max. 23 kg / 50.5 lbs in Economy Class
Max. 32 kg / 70.5 lbs in Business Class
Also, I myself have checked in cardboard boxes with various airlines (also see the Balikbayan box) and there was no problem. It doesn't matter much what you check in as long as it is not too large -- and if it's too large, the airline will ask you to pay for it and then pay more to the airport(s) to handle the problem so you can still check it in.