I’m probably not the first person to notice this, but check out the resemblance between Stan’s mindscape and the Mystery Shack when the portal is activated.
This is both some excellent foreshadowing AND possibly a big clue about how things went down the first time the portal opened. Stan was definitely around when it happened- but I don’t think he was inside the Shack.
Mindscape aside, there’s another potential piece of evidence for this. Stan’s (former) fear of heights, mentioned in Fight Fighters.