If you want a small base, find the location of the Queen.
When going up the stairs, look for a small 2x2m wooden wall piece. Most likely found at the left side, if going up the stairs; about 4m inwards.
Destroy that wall piece and look behind it.

There's enough room for a portal, a workbench, a bed and most likely a fire as there are some openings available.

Thorin :)