Oh yeah that would be much easier and cheaper.
I put duct tape on the black ones. I tried using white paint pen, too. That works really well -- too well in fact. It won't come off.
On grey and green plastic pots, I just write directly with the garden marker. And ever since I found out they come off like a charm with rubbing alcohol, I just wipe off and write on new labels.
I'm refusing to make up permanent labels (triple folded aluminum tape "engraved" with ball point pen) until they are planted in the garden.... For the container plants, I really like the orange plastic spoons though. DH gets them from his favorite ice cream shop. They are re-usable as well.
Another thing DH gets -- rice pudding that comes in these cups because the kids like them. Doesn't it just SHOUT soil blocker? It even has a dimple in the middle for sowing seed. So. I tried it out:
I cut off the rim, which made it flex easily, then wet the mold, lightly stuffed it with pre-moistened mix (not vigorously shoving in the mix like with the commercial blocker with a plunger), then gently unmolded it by squeezing and flexing (just like jello mold)
I sowed some of the pre-germinated cucumber seeds in them. It seems like the perfect size. I also put some too long fermented tomato seeds that started to sprout in the in the middle one. Cucumbers are already starting to sprout: