I was wondering if there are relatively cheap sheets of clear roofing that could simply be bent into tunnel shape and secured that way with iron stakes on the outside along the edges. Open ends could be covered with landscaping plastic or sth. so as to allow easy entrance. The tunnels in my case would be really short, so I presumably would be able to get to plants inside.

I have never worked with this kind of stuff before, so I don't know if it is flexible enough to bend like this. I saw some cheap stuff like "palruf" and "suntuf" panels at the local lumber yard for varying prices, from $11-$36 depending on material and size.

I guess people often make lean-to structures with this kind of stuff?