I am not an engineer and this is no substitute for an engineered design. The attachment could be simply a hole in the rafter with cable threaded thru it and secured back to itself with at least two cable clamps. The hole should be as close as possible to the outer edge, the sheathed side. If the wood is soft you may need to install brackets instead held with several thru-bolts. At the other end you could bolt the turnbuckle but it will pull sideways, unless you add a horseshoe-shaped strap like a yoke. Or use a short loop of cable and clamps to attach the turnbuckle. For cable strength, 'airline' cable, the type you might use on an electric winch, could be good, but it will cut in to the wood, unless you lined the hole with a piece of pipe. 'Strawline' is heavier but will not dig in. Too hard to work with. Cutting cable up in the attic with an angle cutter could start a fire up there. Cut the lengths and rig up the turnbuckles before you go up in the attic. Check ceiling edges before pulling the walls in. The walls might not want to move and you could splinter a rafter trying. In other words, don't even start if it doesn't give easy. Jacking the ridge with screwjacks would help. Conventional wood rafter ties can be installed. all you need is a small hole in one of the gable ends. Even a roof vent can be a big enough hole to bring 2X6's in. Wood is less likely to raise an eyebrow in the future and it's easier to work with. Jack up the ridge, nail in the ties, thru-bolt them or tie them to the rafters with plywood gussets on each side and lots of galvanized nails.