It can be done It can be done with foam blocks if you are careful; nonetheless, I would buy a Malone-HandiRack instead of the foam blocks -way, way more secure, and it can be reused with any other cars.
I do an average of 12/16 races at year that requires driving, and I no longer drive my personal vehicle to most of them -I rent a cars; thus, when I do not find a SUV/Van with cross bars, I use the Malone Handirack.
In fact, I have used it quite a bit lately. I generally drive at 80/85mph for 500/600 miles with no issue. It is all about knowing who to secure your boats.
By the way, I do not carry cheap boats. All my boats are expensive racing carbon/honey comb and the like boats.