Well, I think one ice bottle is sufficient, since you do not want the temperature to go that low. Whats more, one ice bottle will not cool the environment that fast, so it kinda mimics the HL neps environment. Don't forget that when your neps get big, a styrofoam box won't work, cause it unless it is the size of an aquarium.......
On the other hand, you could leave the styrofoam box cover open during the day and leave an ice bottle there. It will ensure that your nep doesn't die out of lack of light and also saves up money, you will not need to buy grow lights. Unless you are growing super highlanders, it won't be neccesary to grow in such a cool environment.
Or, like what Fauzi told me to do, since I want to keep my cephalotus and (Secret Nepenthes) cold, you could use perlite and lay it at the bottom of the styrofoam box. Punch a hole at the side of the styrofoam box so that excess water can drain out. The perlite will work as a cooler and it will also keep the ice bottle frozen longer. Currently, I am using a bucket where I fill up with water, then I put my cephalotus and (Secret Nepenthes) pot in it. I keep it near the brightest place of my house and in the night, I put two ice bottles in the bucket. I am not sure how cold it gets, but I think around 20-22 C, my (Secret Nepenthes) is more close to the water level. It was quite a cheap set up, only cost RM20+.
Cheers