I regularly towed my X1/9 racecars, on open trailers, for about 12 years. In a typical season I logged about 25K miles. I started out using ratchet straps hooked to the factory tie-down points. These are the loops directly under the bumper mounts. That worked fine but it did require that the car be tightly cinched down on the trailer to prevent movement. I used this method for two reasons: 1, it did hold the car very securely on the trailer, and 2, it prevented the shocks from working continuously when towing. This is key when you consider that race shocks have a finite life span before the dampening begins to decay and they need rebuilding. 25K of highway miles, even riding on the trailer, would be the equivalent to several seasons for competition runs and kill the shocks.
For a stock car, using the factory tie-down points and cinching the car down, actually works pretty well. You can pull the car down onto the bump stops and it will ride like that quite nicely.
When I upgraded to an enclosed trailer, the bigger/heavier trailer mean that there was significantly less movement transferred to the car inside. So I switched to using axle straps looped over the lower control arms and not cinching the car down on its springs.