>> Use the deepest pots you can manage.
A 2-liter soda bottle, all by itself, might not be big enough for an oak seedling even though it is fairly tall.
However, you might make a large pot "taller" by cutting the top and bottom off a 2-liter soda bottle, making a mostly-straight-walled sleeve or collar.
Then "screw" the bottom end of the bottle into the top of the potting mix. Go deep enough that it seems pretty stable.
Then fill the soda bottle with moist potting mix so that it makes good contact with the soil in the pot. Plant the acorns near the top of the mix in the bottle.
Probably mostly water through the top of bottle, and maybe cover the gap between the bottle and the walls of the pot with plastic to keep humidity in (but allow some air gaps to help it stay aerobic).
