Well why is simple. He's a fan of rurouni kenshin.

A garage maybe?

its banded together half way, the individual parts are half that size, easy to work on in most rooms at that size.

Do you even outside world?

That would require several days of it being outside(with a possibility of rain).

Just a thing, he might have done it outside but that could be disastrous.

He could make certain parts in his house, and then combine them outside

Always a possibility. But unless he could remove them form each other it would mean he could not use it twice.