More expensive than tunnels, but much cheaper than glass are the awesome Keder systems from the UK. Utilising plastic air bubble sandwich material has been specifically developed to combine high light transparency with good heat insulation, and individual panels can be replaced without re-skinning the whole thing. Available in 2 metre widths, with a welded weather seal this material can withstand high winds and heavy snow falls. I've seen a picture with a couple of burly guys stood on a 2 x 2m section. They have nicely designed frames, with integrated guttering for rainwater collection, good venting and door systems.
The skin design produces a considerable scattering of light (up to 83%) allowing the growing area to be evenly lit, without shadows, so plant exposure to direct radiation is reduced. The layer of sealed air also provides a high level of insulation and heat retention, allowing for longer growing seasons. Keder material is guaranteed for 10 years. I know it's manufactured in Germany, so perhaps it's possible to buy the material from the source. For larger tunnels the company only sell with their own installation, to ensure the guarantee. I have a friend high in the mountains in Extremadura who had one delivered and constructed on the condition he told the boss where the good fishing spots were- so perhaps they will find their way out into Europe more!
It's always worth keeping an eye out for free commercial greenhouses. Twice we've found commercial glass houses being given away and carefully numbered, photographed, dismantled and painstakingly re- erected them. Often garden centres will replace theirs every 10 yrs or so to keep things fresh. It's a LOT of work for a 60 x 10m span, but can save 10's of 1000's of pounds and create very high value growing space. For cool/ cold climates where additional/ extended growing space can be crucial, it's always worth the work.
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