If you only have a few trees, you can set them inside cheap styrofoam beer coolers. They do not seal as air tight as a Coleman or other heavy duty camping cooler. I have added a couple partially full plastic water bottles (2 liter soda bottles) to add thermal mass. The water in the bottle freezes, then keeps the tree cold and frozen on a sunny winter day. The slower the freeze-thaw cycling, the better. But zone 7b is not that cold. Some trees can be left where they grow all year round, fully exposed to the elements.
@EddyFern gave you a great starting list. - I'm in zone 5b, and a firm believer in leave it where it lays. (back trouble has me trying to not move trees around). I leave my eastern hemlock (Tsuga canadensis) on the bench, exposed, where it grows winter and summer. I rotate it, to keep it from getting lopsided, I don't move it. Mugo pines, I don't move mine either. In 7b you should have zero trouble with mugo. No need for fancy winter shelter. Just leave it where it was all summer.
Another species I have found amazingly hardy is Metasequoia glyptiodes - the Dawn redwood. Much more winter hardy than bald cypress, similar look as bonsai.
Amelanchier - any species or hybrid serviceberry is super winter hardy. Often sold in nurseries as landscape material, I never move mine. Hardy into zone 4a or 3b. Nice white flowers, bonsai techniques pretty much same as its cousin the apples, Malus.
Malus - most flowering crab apples are quite winter hardy. I leave mine out in a large 3 gallon nursery pot growing out to size up, it does not get moved from summer to winter. Hardy through zone 4.
Amur maple - Acer ginnala - super hardy. I don't need to protect it in winter at all. No fancy box needed. Japanese maple will probably need wind protection, Amur maple will not. Good reds and yellows in autumn.
But meet up with Eddyfern and see how he winters his trees.