Yes it will be warmer depending on how thick your thick layer is!
Answer: the epidermis of a leaf includes: upper epidermis and lower epidermis but if you are referring to the epidermis of an onion: it is only one layer
chunk
The thin transparent layer of the onion, called the epidermis or skin, contains the highest concentration of flavonoids and quercetin, which are antioxidants. These compounds possess potential health benefits such as reducing inflammation and protecting against oxidative stress. The fleshy part of the onion also contains these compounds but in lower concentrations.
thick layer
it rather depends on how thick a layer you put down.
It is possible, especially if the plastic is a nice thick chunk. If it is a thin layer it may very well be invisible to x-rays but still visible to ultrasound or NMR.
Ozone layer is the thick layer. It protects us from UV rays of the sun.
2000 km thick
The thick layer that surrounds onion cells is called the cell wall. This is common in plant cells and is used to block certain ions from entering.
it is 7 layers
mantle is the thick layer before you reach the core of the earth