I got all my tie down straps made by these guys
Load Restraint Equipment, Lifting Products & More | Load Restraint Systems
The had all the different styles of hooks and ratchets and just made them up however you need. Cheap too.
I was running straps over the front wheels that split around the tyre. At the back I had diagonals that wrap around the chassis members on either side of the diff. I reckon you need the diagonal straps in the back because in the case of an accident. You want some way of stopping the car coming through your back window.
I'll dig them out and take a pic if If you like.
Cheers