Don't worry, she will be fine without any manipulation, forcing her to do strange things with bits of cardboard may actually do more harm than good.
The small rounded green growths, the "cotyledons", are not actually leaves in the true sense, but are more of a protective "cover" for the first true leaves, the serrated ones, while the stem pushes through the soil to the surface. Often the first true leaves are a bit weird, but this is nothing to be concerned over. Each and every seed is a unique individual organic living being, the fact she is a bit slower to take off is also nothing to be concerned about at this early stage, some plants are just a bit slower to get going, even if the are the same strain and the seeds all came from the same packet. Perhaps she is the "normal" one and the other 3 are abnormally fast? There is no need for any artificially enhanced humidity levels either.
Hope this helps,........
Organoman.