You might like to see this photo of a scow I used to carry on a tiny Daewoo Matiz (800cc, 50hp, 1800lbs kerb)
Perhaps surprisingly, this was utterly stable at speed - it would lean over a bit in crosswinds but track straight, possibly better than the car did without it. I put that down to loading it well back so that it didn't upset the aerodynamic stability. Whilst big, it was light, well within the roof rack load.
Incidentally it was a superb fast lane cruiser as well - with a good run up, it would get up to about 75mph maximum speed and the road would empty like the parting of the Red Sea - I reckon most other drivers wanted to get the hell out of its way, assuming it was an accident about to happen.