Jay recced this to me and I think it’s absolutely brilliant.
One little note from the comments though (don’t worry, they were helpful!) - you don’t want roots exposed to sunlight unless that’s part of how whatever you’re growing rolls. This isn’t an issue until you can actually SEE the roots, so it might be awhile.
And easy way to fix this is to cover the top of the bottle with duct tape or some other opaque tape. Another option is a pruning sealant which you can get at Home Depot or Lowe’s (or I’m guessing other hardware stores) and is much better for one’s plants than paint is.
Also, remember that plants are the color of the light wave they reflect! So for most of the time this is green, so if you’re going to use a former Mt. Dew or Sprite bottle make sure that the plastic is trimmed low to the dirt so the plant won’t have any problem getting light.
And while they recommend using 2 liter bottles, I could see 20 oz bottles not being a problem for the seeders. You can always transplant if needed!