What Are Some Ways Ways To Maintain Suitable Temperature In Outdoor Guinea Pig Run?

I was thinking of moving my two guinea pigs outside into a moveable run. My idea is to create some large wooden panels, stretch reinforced chicken wire across these, and nail them together in a large (At least 6ft by 6ft) five sided run. This means that I can rotate it around the garden every few weeks to prevent squalor. Of course, there is always the problem of weather. We have a large, hinged top wooden box like storage unit in the shed, and I was thinking of modifying this as a shelter from the cold, and rain, filling it with wood chips, elevating it to prevent flooding. I live in Ireland, so it can get rather cold in the winter. What measures can I take to prevent the inside of the box from reaching unsuitable temperatures? What's the optimum temperature for the box? I was planning of enlarging the herd, so in essence, I need to insulate the box so that the body temperature of say six cavies could warm it. What measures can I take to make sure the box is perfect. I also know that there can be some problems with outdoor housing, but I'm going to make the run myself, so I can avoid these problems. Could someone give me some advice?