If you have a lot of gear and assuming it's in bags you are probably better off throwing a soft towel straight onto the roof and then tie the gear on top.
Most of the soft racks don't go across the entire with of the car resulting in the entire load being concentrated onto a small area. Most rental cars are cheap and their roofs buckle like a coke can. Never mind that most smaller cars these days have curved roofs. That said quite a number of cars like the Toyota Corolla allow you to fold portions of the back seats. That way you can at least throw your sails / masts/ boom inside the car.
This style of soft racks is a bit better when it comes to load distribution but still not ideal:
harplaneptyltd.businesscatalyst.com/roofrack-systems/watersports-racks/solution-traveller-soft-racksDepending on how many people are travelling with you maybe look into getting either a station wagon or a van. If you look around the net via sites like Kayak you can often find deals with Thrifty / Budget / Europcar that are significantly cheaper than booking directly. Avis also sometimes has a good price on their station wagons when there is an oversupply at a certain location.