I thought about doing something like that when I was young and poor and didn't have a place to live but my car. If you didn't bury them you could drape a camo net over the top.
The sea containers are the way to go, they are solid steel, you could park a semi on top with even noticing. You could lock the doors and the cops would need armore piercing bullets just to make you turn off the tv and wonder if the bullets were coming in or not, but on the other hand if it's all buried and the secret tree was the only way in or out, not much they could do.
I'm sure you'd have an escape tunnel and the ground packed with plastic explosives for a grand getaway.
Sounds like the POT NINJA'S hidden lair doesn't it.
POT NINJA
at my lair
POT NINJA
you can't get me here
you can't get me there
POT NINJA
with super spidey stealth
POT NINJA
run away if you value your health