Before I got my rails, I used to carry large items on my roof simply by placing a couple of the largest foam pipe insulation tubes on the roof & then using a couple of cheapo ratchet straps to hold the item down.
Open doors, pass straps through the car, tighten - job done. Best of all, the whole thing cost just a few quid & could be packed away neatly until the next time.
Had such things on there as Sofa & chairs, single bed & matress, many sheets of 8x4 ply and OSB and 6x4 fencing panels.
Furthest I travelled was 40 miles with the bed on top. It felt secure & I could've gone further.
A couple of surf boards should be no problem.