Just a thought:
Make the tunnels look more like a long house. It doesn't matter how tall you choose to make it, but I'll use these measurements as an example. Put in 4' stakes down the middle of your row then 2.5' stakes down each side a foot or so away. Run a nylon string on top of the length of the 4' stakes and do the on the same on the 2.5' stakes. At the bottom of the outer 2.5' stakes, run more nylon string. Throw plastic or whatever you like on this and secure the bottom to the nylon string with clothespins - or whatever you come up with. That way when global changes happen - you can open up the lower part to get some air.
If I were to build a lower tunnel - that's my initial thoughts.
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