Seedlings and humidity, over the winter

[cythaenopsis]

I have three orchid seedlings (encyclia, cattelya, oncidium) that I've been nurturing for almost two months now. They're doing well, overall. However, I'm worried about the winter time, when I'll be keeping all windows shut and temperatures in my apartment about 55 degrees when I'm gone for the full day.

I've generally heard that humidity trays aren't very effective and that the real deal is to use a humidifier. However, in the interest of saving power, I try not to run access appliances and such when I'm not home. I didn't use one this past winter and the one phal that I had seemed to do just fine (it bloomed in the spring with more than double the flowers of last fall), but I'm concerned about the other orchid species and the fact that they're very young.

Would creating an enclosure for the plants be a good idea, to help maintain a higher level of humidity? I've seen nursery trays, but the dome covers for them are going to be too short for my plants. Also... the domes look to be completely solid, no air vents. So, I'm thinking of creating my own enclosures using clear liter bottles. Has anyone here done something like this? If so, what's a good design that works with adequate ventilation? Thanks.