Heating a tiny greenhouse

When I had a polytunnel it kept out about 4degrees of frost. I grew Tomatoes, chillies, cucumber, fuchsias, coleus and started off many other plants. Apart from the coleus I never lost a plant.
They have their limitations of course but the advantages are greater.
I remember a certain tv gardener who had a ponytail and he grew bananas and fruited them in his polytunnel.
just don't expect them to be hothouses.
as for high winds I was advised to leave ends open which let the wind pass through rather than being a barrier. Structure was fine.
there are of course different grades of polythene and frameworks and as in many things cheapest is not always best.
mine was from Citadel in Crediton Devon and good service etc. Not sure if they are still in business but you can always check online.

A TRUE gardener adapts what he/she has at hand.