So then my fellow robo-hunters, if you're one of the unlucky gamers and can't get the Astronaut Helmet to spawn try the following ~
After reading the other Solutions I replayed Chapter 3 - Metal Slag Trail - Level 3 The Factory for 4 hours straight (3 hours solo, 1 co-op) and got a shitload of nut boxes, but no headpiece.
Since all of Chapter 3 are classed with level 12 - 15 items I started trying the level before the Factory called Highlands, also in Chapter 3 Metal Slag Trail.
Bam, first try, first box. My friend who I did the co-op with had the same issue and he got it on the first box after hours of the Factory with no luck there on Highlands aswell.
Hope it helps, and if you have the E.V.A. Suit, equip that with it. I read from some others the other matching piece makes it spawn faster.
Happy hunting, here's a puppy if you're having trouble with the Achievement so it eases the possible annoyment.
If you still have problems... you can always buy it for 99 cents on the marketplace / in-game store.