Do you have to cut 'em?
How 'bout if you made a square- 3 tires by 3 tires - plant inside the square also inside each tire.
I wonder...that might could work. Wouldn't quite make a square, but it would be close. Could make a nice bed! Could plant marigolds and such in the tires and veggies in the bed.
I think the only concern would be making sure they were filled with dirt, or you get skeeters.