What zone are you in? Sango kaku can be picky about wind and cold....

Zone 9. The middle of my garden is enclosed by deciduous trees in all directions. it will get plenty of protection in summer but not winter.

My main concern is not the trees health, it grows quite well in the pot where it is, but its longevity. It's mainly about the concept of a garden bed for a tree like this. Is building a raised bed a bit like having a tree in a big container that happens to be on the soil or it is considered more as part of the ground but just higher? I mean, if its the former and if I make a bed the size I described what will need to happen for the trees roots to have enough room to sit in there for 20 or thirty years?

I suppose it would do alright as long as the box didn't have a bottom and it received the necessary water.
Why do you want to put it in a box in the middle of your lawn?
Mike

There's no place to put it to be honest. The surrounding garden beds are full of invasive suckering trees and the maple probably wouldn't compete.

That reminds me of another japanese maple planted in a box with no bottom. I think the pic was posted on this forum a year or two ago.

Wow ... the will to survive!