Measure the cat stretched out, then ignore most labels
Maine Coons outgrow standard carriers while still young, and they keep filling out for years, so shop by your cat's actual measurements: length from nose to tail base, height at the shoulder, and his weight today, not the kitten you remember. He needs to stand without crouching, turn around, and lie stretched on his side, which usually means an extra-large or dog-sized hard carrier, whatever the cat section of the shop suggests. A carrier bought snug for a young Coon becomes a costly mistake within the year, so buy for the adult frame his parents and growth rate predict.
Structure that can carry the weight safely
At his size, the carrier is luggage as much as bedding: look for a rigid shell, a handle rated well above his weight, and ideally two-person or two-handed carrying options, because lifting a large cat in a flexing box is how doors pop and backs complain. Wheels or a trolley design can help for vet corridors and stations, provided the ride is smooth and the carrier stays level. Check that door latches engage at multiple points and that ventilation panels are metal or heavy moulded grille, since a Maine Coon's casual lean carries the force of a small dog testing the exit.
Two doors make life easier for everyone
A carrier that opens from the top as well as the front is close to essential at this size: lowering a large, possibly unimpressed Coon in from above is safer than pushing him in headfirst, and at the vet he can be examined in the base without a tug of war that stresses the whole appointment. A removable top half also doubles as a bed at home, which is the easiest way to keep a big carrier permanently familiar rather than stored in a loft and produced smelling of dust on appointment days only.
Airflow and space for a heavily coated cat
That magnificent coat overheats in a stuffy box, so choose generous ventilation on multiple sides and avoid draping the carrier completely, however tempting it is to keep him calm; a partial cover that leaves airflow panels open is the working compromise. In the car, position the carrier where air reaches it, belted flat and level, never in a hot boot space. For long journeys, plan a larger travel pen arrangement at stops only in a locked, private space, and use a clip-on water dish, because a Maine Coon drinks seriously and spills enthusiastically in equal measure at this scale.
The home routine that makes the carrier work
Leave the carrier out permanently with bedding he chooses, feed occasional meals in it, and practise short lifts and car sits so the whole sequence is boring long before it is necessary. Check his growth against the carrier every few months while he is young, the same way you would check a child's shoes. Travelling with his full history in his KinsLe Pet Passport means any vet, anywhere, can see the records of a cat whose size alone raises questions worth answering from paperwork.