Well, my new guinea pig nursery that I built inside is working wonders. It is protected from elements and stays around 40 degrees at the lowest this time of year, as it is on the mud porch.
Any guinea pig that looks eggplanty, comes inside to birth with guinea pigs that have other young. Here, they stay until the guinea pig babies get to be about 0.5 lbs. This way, food is guaranteed to be the best of the best.