I was driving a van that was lifted by a tornado, another time almost drove into one if grapefruit sized hail hadn't made me turn around minutes before it touched down on highway, that's one where I met caravan of "Storm Chasers" afterwards, disappointed because they didn't get killed, or even see storm, fools. Have been within 1/8 mile ot two others. 1/4 mile from my home twister touched down , demolished many homes 4 blocks wide over a mile long. Did usual twister tricks, Brand new home built to new stricter storm code, completely leveled, next door western ballon frame home untouched. Last thing it did B4 going back up was suck heavy metal 8X8x8 ( a small shipping container) box from under Scout masters carport, our troops camping gear was long gone including four large cast iron dutch ovens. Box was found a mile away, ripped open and crushed.
Grew up in Texas Panhandle. many storm cellars in area. One of first things settlers in plains did was dig a "fraidy hole" They may have been used as root cellars but that was not purpose. Best place to be in twister is under ground.
During Cuban Missle Crisis every vacant piece of land large enuff and Shopping center parking lot had fall out shelter on display. ( Home town had SAC base and Pantex where they made them atomical bombs, we "knew" Russkies was gonna get us. ) Shelter was small quonset hut to be burried in ground.
Buried shelter of corrugated culvert is inexpensive, perfect shelter, doesn't even have to be very deep, 3 or 4 feet to top of pipe. especially if under reinforced concrete slab used as patio. Two inward opening hatches, sort of like airlock, another exit ten or so feet down a culvert tunnel is additional.
last i saw on above ground safe rooms they were in the tens of thousands of dollars. Even here in Tornado alley, few folks even have cheap fraidy hole. I think,due to randomness of tornado action, they "Watch the skies" and take their chances. ( More folks get killed in traffic than twisters, but they still drive like maniacs.) And after all if hiding under school desk protects you from Commie A-bomb, cardboard box should protect from twister.
Shipping container, if buried with proper exit, would be good shelter. Not so good on surface. Aside from projectiles and cars from parking lot being dropped on it, bolting down would require as much excavation as burrying it. Also look at pics of containers after being storm tossed at sea. They are not as rugged as they seem.
Here's vids of semi trailers being tossed a few miles from me, last spring. You can turn off sound to avoid mindless news anchor chatter, or listen to my town being mentioned a few times.