I bought a second-hand hutch from ebay recently in preparation for getting two piggies. It's 4' and has proper catches on the doors, but now I've looked at it closely I've realised it's made of very thin wood. The frame is more solid but the walls and sides are made of plywood. Is this going to be ok for guinea pigs? I'm planning to cover the floor with a sticky-back plastic sheet and to re-felt the roof. Do I also need to add another layer of wood to the outside walls and floor to insulate it a bit? We're intending to bring the piggies inside in the winter, but obviously there will still be cold days in spring/autumn.