Hi Ian... I have a couple good sized greenhouses and a few suggestions.
Wait until your hotest day to stretch your film over your frame. This is
the best way to pull the film drum tight which minimises its movement and
stops fatigue wear. A tight skin also stops heavy rain from puddling on the
canopy.
Use treated timber for your battons and consider screwing them on - screws
have more holding power than nails and they will allow the battons to be
removed and recycled. Besides it's a perfect excuse to buy an electric
screw-driver.
I would warn against using carpet in the way you are considering because
condensation is inevitable on the inside of the canopy. Carpet will act as
a sponge and become a hot-bed for mold, mildew and fungus disease.
Regard Ian Gill
Westland New Zealand