The final look for the bomb was realized by the outstanding craftspeople at ISS Props.
The design was based on an actual Cold War man-portable Nuclear device. The SADM, Special Atomic Demolition Munition was a family of nuclear weapons fielded by the US military in the 60s. Thankfully never used in combat, these were designed to destroy, irradiate or deny passage. Not to mention that the yield on one of these devices essentailly made it a suicide mission for the Engineer in change of placing such a munition.
The case was designed to look like space cargo, straps and buckles everywhere.
There was exploration of small demolition charges as well, since most spacecraft is more like a tin can than an armored submarine, where a small explosion would destroy anything fielded by man so far. “Nuclear lunchbox” was the initial prompt. And after some reflection, considering Roy has to propell himself back to earth, an antimatter explosion triggered by a nuclear bomb, seems to make sense.