It was gurgling, before I added more cotton. Now it's a tight draw, but still leaking, occasionally spitting back into my mouth. Should I still try a second cap?
No; if it wasn't sealed by the silicone cap it would gurgle.
What internal diameter coil did you make?
I made protank coils for months, and near the end I discovered it was better to wrap on a bigger diameter screwdriver that wouldn't fit in the slots, then use a smaller screwdriver that fits in the slots to mount the coil; then I could put more cotton inside the coil; and that way I don't need to put extra cotton on top;
that extra cotton on top tends to expand and block airflow, so the more cotton you can fit inside the coil where it is kept contained, the better; meanwhile you can still add a thin topwick and not a lot of topwick that might expand and block the airflow.