Prepper Stash: Foxhole
This stash is at the Armstrong Residence in John's territory, south west of US Auto. Note this is another stash that's easy to access after you've completed it.
Here you'll find a burnt-out house, and probably a few milling cultists.
Clear out the latter, and approach the former. The note is on a trunk outside the house. Read it to start the mission.
For a burnt ruin, the house is surprisingly hard to access. Start by jumping in through the plot-markered window (the water heater will blow sky-high as you do).
Turn left (north), and head through the doorway.
Turn left again and jump over a mildly barricaded door.
Take the ramp up over the flame.
Drop down from the ramp facing east, and hug the righthand wall. It'll take you to a table+shelf combo you can use to jump back up to the second level.
From there, jump across some roofing sections to the northeast corner of the house.
Head south to the bunker door. Just as you draw near, the water heater from earlier will land, knocking the bars off a nearby window.
Enter and loot the bunker to complete the mission.
Note that there is an arcade poster outside here.