I don't have a lemon soda recipe, but I recently made a lime soda (I tend to think of lemon as that ingredient that ruins sprite by perverting my beautiful lime flavor).
I don't know what kind of batch size/process you're planning to use. I made mine directly in an empty 2L soda bottle.
I had some leftover lime extract and lime bitter I had made a while ago for a beer recipe and decided to use them up in this. I used a lot more than I had expected to need, and I still didn't feel like I got enough lime flavor to come through.
Mostly, though, I felt the soda came out flat. If I were going to do it again, I would definitely consider upping the acidity of the soda. Mine had no bite to it. I used 4 tsp of lemon juice in 2 liters, which is sufficient for acidity in my ginger ale, but was not nearly enough here.
I don't think there's any way you're going to pull it off without lemon extract, even if you just make it yourself (lemon zest + vodka for a week or so). After that, just taste it as you add it until it tastes right.
If you'd like, I can dredge out of my memory what I used to create my soda, along with what changes I would make it if I were to make it again today.