g-max box dummy?

I need some help on how to make a dummy part or invisible box that surrounds a airplane, that is called interior or exterior. I know how to make a normal dummy but When I downloaded a aircraft from freeflightdesignshop, I went to group then went to open, and that their for let me able to model the object. I want to know how to create that box. Also, when I played the animation, the gear pushed down the box. So much for that, anybody that might have an answer?
Thanx.
Thanx.
