Depending on the size of the pots, you could always dig a hole in the garden and sink the entire pot in the hole. The soil will serve as insulation and it will be as if the plants are truly planted in the ground.

If the pots are easily moved, then you have several options. You could move the pots to a protected corner outside and pile leaves or bales of straws up around the pot to offer more insulation. You could leave the pots outside until the plants have gone dormant for the season and until it starts getting below freezing and staying that way for extended periods of time and then move the pots to an area where it will stay near but above freezing so that the clematis remain dormant the rest of the winter. This could be an unheated garage or even basement provided the basement is not heated. I personally would never try to keep a larger clematis actively growing over the winter since it is likely not get get enough light and start getting leggy and more prone to get infestations of various critters such as mites, aphids, etc. If you have a greenhouse or a solarium where there is plenty of light, then I would try to keep the plants actively growing and only then. JMHO, someone may have other ideas.

Since you are in zone 6a they may be just fine where they are. Clems are very hardy. I'm warmer than you are so don't take my word for it but I would not be surprised if they are fine.

Plants in pots are less hardy than plants in the ground but most clems are hardy down to zone 4.

You could wrap the pots with bubble wrap.