This update allows you to directly change the item.
/setwarp <name> (color code) (item)
/setwarp spigot 5 dirt
Using this command will create the warp spigot in a purple color for gui looks with the icon as a dirt block.
If you mistake the item like trying to do endstone instead of ender_stone it will create the warp as normally but will notify you about the item being wrong and create a barrier for it instead.