Upvote:1
While plastic boxes will likely be accepted, well sealed and reinforced with tape on all edges, it is up to the agent handling checking in for the final decision.
If you pack the boxes in bag, and those can be really cheap, it is much less likely that they will be rejected.
And such a bag will protect your box and keep the contents of the box together in case a box does break.
If you had not already bought the boxes I would have advised you not to buy them but go for duffel type bags or suitcases, as those are much sturdier.