Hi Andrea, I think that plastic greenhouse will hold no frost at all, also it
is far to expensive, I think.
Can't you build something against a wall of the house, it has not be too
big because the cultivars you have are not the tallest bananas.
It will get a fair amount of heat from the house too.
Here 2 examples of additional heating you can use.
But ofcourse any other heating method will do.
Heat-pads (35 watt)
A very antique petrol cooking device( not for sale
)
Use thick styroform on the bottom of the greenhouse and for the greenhouse polycarbonate-glass. Make sure there is a window at the
roof of the greenhouse wich you can open/close to release the heat in spring/summer. You can use bubble-wrap on the inside in the winter.
Now, it will stand 20F without using the heath-pads and petrol heater, at least in my case.
So, this setup is good enough for my zone but don't know how cold it can be in your.
They surely sell those greenhouses somewhere and it is a better investment
than that plastic thing.
Ron