I was worried about my new clematis when I left them outside in pots. The pots froze solid in the first cold snap we had and I thought they were a gonner. Last week a spotted new growth so they are very hardy 👍
Yes they are very hardy and the cold makes them that way. The box planter you could line with some black polythene perhaps unless anyone else has any ideas!? I think you also need to shade the first few inches of clematis growth it really helps. Some seasonal bedding would probably do the trick. Also a deep pot would be ideal.
Agree with @GavJohn - and black poly is as good a liner as any. Remember drainage too - broken up polystyrene works a treat. Plant deep - deeper than you'd think and bury the first few inches at least of the stems. Honest it works!