I have used a similar set up for the past few years. White overwintering plastic film reduces daytime solar gain. It's very important to prevent excessive heat buildup to prevent the formation of tender, new growth. A small emergency heat source may save vulnerable trees on the few, out of ordinary cold nights. It's again important to not overdo this emergency heating.
Night time temperatures can be below freezing without harm if the plants are dormant. venting needs will largely be determined by the amount of sunshine. Trees planted directly in the ground will not be as vulnerable as potted trees. Slightly moist soil will retain more heat from day into night, especially during sunny days.
Further tweaks include insulating between the inside and outside soil at the perimeter, using water barrels as heat storage sinks(this can be a disaster if the water freezes).
There are also seasonal white shading sprays that can be applied to rigid plastic and glass surfaces to reduce solar gain.