The issue is how to avoid a huge amount of ski extending beyond the back of the car. A problem is that the typical roof rack crossbars on a car will be positioned such that the front one is about half way along the length of the car and the rear bar is near the back of the car. Another problem is that the distance between the crossbars is relatively small compared to the length of the ski. The latter can be solved by buying a purpose built ski support rack like this. However, when positioned so that the ski extends over the front of the car as well as the back, the weight is just about all on the front crossbar. This results in the whole setup flexing and this can be disconcerting when travelling at speed.

I've introduced an extra support at the front, from the car towing eye to the front of the ski. This goes a long way to supporting and stabilising the surfski.