Since you already own the neo-air, try stacking a blue ccf pad ON TOP of the neo-air. This is the approach that works for me, I've used it down in the the upper teens and lower twenties.

What I use are thinlight pads from Gossamer Gear. They used to sell 1/4" thick pads, but they still do sell their 1/8" thick pads.

I like having three options of ccf pad thickness (1/8, 1/4, 3/8). Based on expected lowest temp I can mix and match to combine one or more of these with my neo-air. So for example, I might put a 1/8" thinlight below the neo-air and a 1/4" on top, or in really cold conditions a 3/8" on top. Definitely put the ccf pad on top of the inflatable for overall best r-value.

I certainly don't certify this for anyone but me. Too many dynamics to recommend a system for someone else: tent or other shelter type, sleeping bag, what clothing you wear inside the sleeping bag (or quilt), personal metabolism and physical condition, external temp and humidity, ground temp, wind, just where you pitch your tent (local site selection can make a big difference), ... that sort of thing.