Best would be to bury the container to the soil line against the north side of a heated building away from sun and winter winds and mulch well. Some protection like a burlap wrap would help to guard the top against dessication if the site is windy.
Not so preferred, because the tree is guaranteed to leaf out too early in spring, is to overwinter in an unheated garage. If that happens you'll have to do the "Acer two-step" (move the tree in and outdoors as temperatures allow) until danger of frost has passed to prevent the long internodes and weak growth that will occur in the garage and destroy the trees form by radically altering its normal growth habit.

Al

Here is a link that might be useful: Additional comments here

Sharon i've had excellent success drilling holes into a lg rubber trash can then stacking my containers inside and finally filling it up/in with fallen leaves. I leave the container uncovered and check it every so often to make sure it is draining. We get close to 0F here most winters.

Even with dozens of 1 year grafts i hardly ever lose a plant.

Herman,
Can you take a picture of this setup and post it here? I am curious as to what you've done.

I have 3 Japanese maples in pots right now (all 1-2 year old grafts) -- Aconitifolium, Orido Nishiki, and an unnamed cultivar that was growing in my brother in law's yard that was a seedling from what appears to be a bloodgood, but I could be wrong about that one.

So you folks recommend burying them in the ground on the north facing side of my house?

Thanks,
Tony

Tony, i didn't know NJ had a zone 5. I'll take pics when i do it maybe in Dec. It's simple as described.
Use a 30 or 40 gal trash can, drill lots of holes in bottom sides. Put an few inches of dry leaves in bottom and start layering plants in. About 6-10 1gals fit in a can. Use plenty of dry leaves for insulation throughout. Keep it uncovered, it is OK if the top branches stick out of the can. Works great for me, be been doing it for 5-6 years. I have no garage nor any place to bury--very urban here. Last year 100% survival (3 cans.)

Thanks, Herman. That sounds like a good idea. And you are right, I think the zone stuff was mislabeled when I first started posting. So you put 1 gallon pots on 1 gallon pots? Interesting idea. I guess it's easier than digging 6-10 holes?

I have a garden in Cape May NJ and there it is a toasty Z7. But in philly it's all concrete and a lil bit colder. Ya, all of them aren't in 1gals but they are all small enough to fit easily. I stagger them and stack them, there is still plenty of space for the leaves. In the spring i lift them out. Sometimes the bottom layer might be frozen solid even in mid march. I just let it defrost and no issues at all so far. I suppose it's easier than digging holes but that isn't really an option here. In my home growing up i used to place them in a corner of the yard and stack a huge pile of leaves on top, then some stakes so my dad would't "clean out the yard". It probably worked as well but i was less tuned into the plants needs and i lost a few.