I was wondering what Doomguy's helmet is made up or or is based on: for example, a children's helmet. Do you know the origin of Doom's helmet?
This particular cover art was created by Don Ivan Punchatz, who sadly passed away in 2009. It's an original work on his part. As for where he got his inspiration, one can only speculate.
You should also note that this is not the only rendition of our space marine hero, see for example the PSX boxart and art for DOOM II (though one could speculate he's the same guy sans-helmet as he's now back on earth):