If you're going to make one using threaded rod, make sure it's a minimum of Grade 5. Standard hardware-store/Fastenal stuff is usually Grade 2 at best; anything beyond that usually requires a special order. The Grade 2 stuff will break under standard flexing of the factory motor mounts.
Side note: if you're using spool-type motor mounts, a torque strap is essentially unnecessary. Either use polyurethane inserts or just fill in the voids in a factory-style rubber insert with black silicone. Either way is plenty strong but the poly option will introduce much more engine vibration into the interior, just like a taut torque strap will. Using only one poly insert would lessen that and since the passenger's side mount is constantly under compression there's no real need to beef up that side.
Of course if you feel you really need a torque strap you must be building a pretty serious combination, so vibration and daily-driver comfort is probably not a concern anyhow.
