Joachim,

If you could turn the yak upside down, you wouldn't need to cover the holes. I would think a light bulb would suffice. I've heard tale of folks heating dog houses with a single light bulb, so a yak should warm nicely with one. As with anything, test it. Just how cold is it outside the yak?

I've used drop cloths to tent the yak, then a small electric heater set on low. Some folks elect not to use electric heaters for fear of burning down the place, but just keep the flammables a safe distance away. Remember, heat rises.

Hope this helps,
Danny