Purity: Covering the Private Parts During Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 591:
During the prayer, the private parts of the corpse must be covered. If it has not been possible to shroud the corpse, the private parts must still be covered, albeit with a board, brick, or something similar.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)