My younger brother checked two good sized cardboard boxes ... probably 30x30x30" each ... both weighing close to 50 pounds on a flight from Boston to LAX last month. No issues whatsoever with check in. Did it for the same reason as you. Filled them with clothes and other items he needed for a move. Nobody said a word about it, accepted them just like they were a suitcase. Though they could of course get damaged, they came out just fine.