Airtight Hangar Door
From Space Engineers Wiki
The Airtight Hangar Door block is a 1x3x1 block that can be opened and closed, designed to be used in groups to create large entrances to hangar bays where ships can enter and exit, while still being able to contain oxygen when closed.
When open, the block takes up 1x1x1 in size. When closed, it extends to 1x3x1, sealing off two blocks worth of additional space. Two can be placed opposite each other, leaving a gap of four blocks as an entrance when opened. Any number can be placed in a row, giving you an opening of 4 by any size as a maximum hangar bay opening without resorting to more creative measures or mods.