I have only read LU, not played so I did some googling.
The A5e
blog says "A haven is a place where you can rest, get a meal, a full night’s sleep, be protected from the elements, and are at little risk of attack. An inn is a haven; a campsite is not (although you can still take a long rest at a campsite—you just can’t recover fatigue or resolve). Some characters may be able to create havens in the wilderness, at least temporarily."
The srd has:
Vehicles: "A vehicle acts as a haven if it provides accommodation and shelter from the elements. At the Narrator’s discretion, the environment may not be suitable for it to act as a haven."
Encampment Stronghold feat: "At Grade 5, your familiarity with temporary structures is such that you can spend 1 hour to create a haven able to shelter a number of creatures equal to your proficiency bonus. You can’t use this feature again until you finish a long rest."
So based on that, it's plausible to get a proper rest on a ship or a traveler's wagon or a camp made by someone who is highly skilled at living outdoors.
There's plenty of wiggle room (pretty sure a dog sled or a canoe doesn't work as a haven)
Assuming the requirements are protection from elements, a level of defensibility notably greater than a simple tent, and some vague "accomodations" that might be buildable in an hour.
So based on that...
Meld into stone: you are ensconced in stone for 8 hours. "No trace of your presence is visible or detectable by nonmagical senses." You clearly have no concerns for the elements and attacks are between minimal and impossible, as with something like a mountain or just the bedrock, it may impossible to break enough rock to force you out in less than 8 hrs.
Wall of Stone: If you are in a cavern it's pretty easy to put up a barrier that is maybe 5ft of solid rock. Just put some narrow cracks at places for air. And have a plan to escape later. But you can probably sleep like a baby for 8 hours.
In a more traditional form of shelter, with rocky terrain (or a shovel to dig a bit) to anchor it, you can make a 40'L x 20'W x10'H box structure with 3" thick walls or more useful barn shape at 20'L x 20'W x 20'H (10ft side walls, arched roof) with a 10x20 panel to use as a loft, a "porch" to protect horses or curved wall in front of the door to protect from battering rams. Shape a few narrow windows/arrow slits, a simple chimney, and you just need to make or haul around a decent door.
Won't protect you from an army or a dragon but most wildlife will be baffled, as will quite a few bandits who might decide "it's not worth it"
And c'mon, 3" of stone have to be better than a wooden wagon or whatever it is Ranger Rick is able to whip up in an hour.