The Signature All Metal Pajama Man Lunchbox (or Portable Bad-Guy Containment Unit by Sam) is an item that Pajama Sam uses in Pajama Sam in "No Need To Hide When It's Dark Outside". In the game, Sam planned on containing Darkness within this lunchbox after he beats and captures him.
At the start of the game, Sam needs to find his mask, lunchbox, and flashlight in order to capture Darkness. To find Sam's lunchbox, he should find it either under his bed or in the wastebasket. Once he finds all his things, he enters the Land of Darkness.
Unfortunately, he soon gets caught by a snare trap by the Custom Inspection trees and they take away his things since he didn't declare these as his when he entered the Land of Darkness. The lunchbox can be found in two different locations: underwater in the underground cave or with Wishing Well in Darkness' garden.
In the Underground cave storyline, Sam will find his lunchbox underwater in the cave as soon as he and Otto go down the waterfalls, however Sam can't reach his lunchbox by hand as it's too far down for him to reach. After Sam gets the magnet, he can use it to pull his lunchbox to him and reobtain it.
In the Wishing Well storyline, Sam will notice his lunchbox with Wishing Well as soon he gets past the trees and into the garden, however he can't get to Wishing Well by foot due to the various thorny vines blocking his way. After Sam goes to the underground cave and gives Otto some oars, Sam will be able to reach Wishing Well's bucket from underground and reach him, allowing him to get his lunchbox back.
Once Sam has all his things back, he will go to Darkness' bedroom and closet and confront him in order to capture him, however he soon realizes he doesn't need to do that, as Darkness is a friendly character who only wishes to have a friend to play with (he is unable to do this because once he's awake and out, everyone else is asleep).