Exciting – yes your idea is good. Although at this time of year, with light increasing fast, tunnels have a wonderful effect just on their own without ant supplementary help. We already have as much light as in the middle of October. Daytime warmth is good, the problem is at night when your extra tunnel will be most helpful.
You can sow radish, spinach, lettuce, spring onion, calabrese, carrots, beetroot…. but wait to sow frost sensitive vegetables.