Here's a short and informative article on growing clems in pots.

Whiskey barrels will rot over time, and since the clem will (should) have the rooted area in shade it could rot faster.

They don't take up much room in the ground and with our freezes, I'd put them under ground.

And they prefer cool shaded root areas, much easier to do in ground.

Why does your vision want it in a container anyway ? Do you want to move it around? A barrel is too heavy for that (imo). Plus clematis stems aren't easily untangleable from their support, they break easily/fragile.

Want it for the arch. already have a clem growing in the side where there is dirt. Arch reaches across a concrete walk that is up against a garage.

Well typically clematis follow the sleep, creep, leap rule (3 yrs to full production). So about the time the clematis is getting going good, the barrel is considering turning into wet rot. Good luck though.

Well maybe plastic would be the way to go! Thanks