A stack of about 50 small 'cardboard' seed planting pots (Broadstone BH18)
We bought these a long time ago and they worked well. The pots are designed to be planted out with the young plant in them, I tended to break the cardboard as I put the seedlings into their permanent home, but the roots managed to break through the cardboard anyway.
This offer has been gifted to someone and is no longer available.