2 Liter Bottle

Instead of starting plants indoors you think starting them in soil with a 2 liter over them (with holes for ventilation) would be as good as a fence until they get big enough.

Fence? It acts more like a green house to keep it warm. I read that using a one gallon milk container works good to extend or early start a season so i would think you could use what you have instead. It said though to just remove the cap and that would be enough airflow cause the main purpose is to keep the plant warm at nite and protect against freeze so too much airflow could be fatal. If where you are gets cold at nite you can put rocks around the plants to absorb heat during the day and they will slowly release it at nite. or use mulch to generate natural heat.