Depending on how far off of straight the tires are when the steering wheel is locked. it might just be simpler to lash the steering wheel to keep the wheels straight. Before we bought our Edge, we towed a Chevy on a dolly. The steering wheel did not have a lock so we just lashed it to the seat mount with a ratchet strap which kept the wheel from turning.
1999 Newmar MACA 3796 F53 6.8L
2016 Ford Edge Sport
Roadmaster Sterling A/T with Brake Buddy Select