Adult pure super dwarf retic. Yes the cage is narrow but the snake will use the extra height, as retics love to climb.
If you want a smaller snake something like a Dominican Red Mountain Boa, or one of the smaller dwarf locality BCI's can be set up in it for display, they're semi-arboreal. Another option is the thinner pythons, like a diamond python or one of the smaller liaisis species as they'll also use the height if you give them something to climb on.