transplanting

So I started a couple seeds, and I made (what I believe was a) Mistake, by planting one in a 1 liter soda bottle. It's one of those awkward shaped coke bottles.

I found out that it's not a great idea because there isn't enough room for the roots, to reach any decent height for the plant.

If I have to transplant I'm curious what the best way of doing it would be, where the plant isn't just going to slide rite out, and there doesn't appear to be enough root structure yet to hold the mass of soil together with the roots if I were to cut the bottle open or something along those lines.

thanks,
sponge

In a week or two, when the root mass is tight, cut the bottle with heavy scissors or tin snips.