"Cascade Ice" flavored sparkling water comes in a 17.2 fluid ounce bottle that's 2.5 inches in diameter and 7" tall in usable height. It has completely straight walls with no ribs.
These are a little bigger than the 6” X 2” "test tubes", but not big enough to be used for potting them up.
A 2 liter soda bottle is 4.25" in diameter and 8" tall usable height. Some have straight sides. Those 2 liter bottles can serve as "potting up pots" for either 6x2 or 7x2.5 seed-starting cells, especially if you don't fill the seed-start-cells all the way to the top. However, if the walls of the seed-start cell block the seedlings' bases from the fan or air movement, that may increase humidity and damping off. Keep the soil surface dry!
And empty water bottles and soda bottles are free!
