I've see plans and pictures of shelters where the bench seating was a series of plywood boxes. A box about 14" across the top and 18" tall makes a good seat. Fitted with hinge along the back you have a considerable amount of storage capacity. Figure at least an inch of overhang and raised on legs or dunnage to keep the box off the floor so it doesn't collect moisture and a tight 16" of linear seating per person you end up with an allocation of a space roughly 2688 cubic inches(12"x 14"x 16"), or a bit more than 1.5 cubic feet of storage per person. Collectively such bench seating type lockers give you a significant amount of storage.

I've seen at least one plan that used the void in a tapered backrest to hold a simple rack for one or two long arms with space above for a pistol hung on a peg and narrow shelves for boxes of ammunition and accessories.

You may want to keep one bench seat unoccupied by lockers or supplies so the family dog has a place to lie down without being under foot.

Also the typically unused areas above doors and above the shoulders or heads of seated people can be exploited for cabinets, lockers and shelves.

Planks can be placed between seats or up on seat backs to create a place to sleep and/or hammocks can be rigged off rings in the walls or ceiling.

Pegs or hooks placed along walls, possibly above the seat backs, allows people to remove and store bulky coats, rain gear, and hats.

For a tornado the accommodations needn't be much more than sardine-tight standing-room. Uncomfortable as it may be your only going to be in there perhaps thirty minutes at the outside. Hurricane shelters need more sumptuous accommodations because people may need to be inside for 24 hours or more. Consider the probable duration of the risk when planning per-person space requirements.

I have seen a simple 4' interior diameter concrete sewer pipe dug in and upright with a ground level reinforced slab poured over and flush-fitting 2" square hatch fitted. Inside was a bench seat circling the outside wall that gave comfortable seating for at least three. I can't imagine how they planned to sleep, perhaps curled up with a lot of spooning, but for short term use it would be fine.