My personal headcanon is that Daniel has developed a form of claustrophobia after being turned on the private plane. Much like my own experience, it's not strictly the small spaces, but the feeling of being vulnerable without means of escape. So the tour bus is mostly fine, he's still on the ground and could easily mind-fuck the driver to stop or jump out a window if need be. But, being in that plane, no exits, no safety, all at the whim of his maker... not to mention how harrowing the dying process must have been for him. Alone, trapped, and terrified. That intensely traumatic experience is what planted this fear within him.
We also see this type of claustrophobia with his purple sleeping bag. A coffin is too risky, as he's in a prone position, defenseless to any predator. Instead, he chooses a sleeping bag in an upright position. His eyes and ears are exposed and alert, whilst the fabric makes for an easy get away.
He doesn't show it often (or at least- he tries not to), but Daniel is a very anxious and defensive man, who feels like he can't trust anything or anyone after his making. We already see it in the way he communicates, but the little details like these lead me to believe he's suffering more than he portrays.
