2 liter bottles work great for humidity domes. I actually germ in 2 liters too. I quickly transplant as soon as the sprout has established, cause more times than not, sludge build up in 2 liters occurs. Maybe cause the bottom isn't flat, I don't know. I've tried different types of materials and rocks in the bottom. Seems to happen anyway.
Only other downside is unless the bottom is blacked out, your root will shoot straight down with a fucking quickness.
But I never have a seed not germ within 48hrs using 2 liters. I didn't have black spray paint for my first few a while back when I started growing, so I used black trash bags cut to size and taped. Black spray paint makes the job easier and quicker (unless you count drying time). Keep your bottle around 3" in height. Any more and like I said earlier, your root will become too long unless you transplant directly into your flowering pot. Use your cut off top for humidity dome from plant to seedling. Never put the cap on. Excess heat and stale air needs to get out some way. You can guesstimate your humidity by how dry the dome (top) looks. Mist it when it's dry. If you watered the medium and let it sit to get moist before planting your seed by misting the top of the dome and leaving it on at all times will keep your medium moist as well. If temp gets above 85 F however, you will then have to take the dome off so that you don't get the medium too hot (too much heat in high temps is great for mold growth, which you don't want).
Try to cut holes around the bottom on parts that are slightly elevated off the ground as well to allow air flow at the bottom. Helps force bad air and excess heat through the dome.
If you need some visuals, I still use this method on my non-experimental seeds and grows. Send me a PM, and I'd be more than glad to send you some pics.
On another note, they work great for germing tomato plants as well