Have you tried calling around to place's close to you there in Ohio that might sell them or looking online at Ohio store's that sell tree's?
Maybe you could take a bunch of rope and tie it to the roof of the van.Run it over the box and through the window's several time's . I would say use bungee's to tie it down ,but going a long distance I wouldn't trust bungee's.
The reason I don't trust bungee's...when I was little a bunch of us went on a trip in the family station wagon . Had luggage in the back and some on the roof. While going down the interstate one of the bungee's broke , a suitcase came off . A semi behind us ran over the suitcase which contained our toothpaste,shaving creme ,Noxzema,etc...needless to say it was not a pretty sight.