Last spring I drove my Kia Spectra to Florida, and I strapped a long board to the top of it without a rack.
This is what I did (it cost about $20 total)
Buy two good ratchet straps.
Buy one Foam Noodle (The long foam noodles for... Ok I really have no idea what they are for, I think they are for playing in the pool or something, they are long foam noodles that come in a bunch of different colors, they are usually sold in WalMart, in the toy section.
I cut the noodle in half, so I would have two noodles.
I laid the noodles on top of my car (across), one over the drivers/&front passagers doors, and one over the back doors.
I laid the surfboard on top of the noodles, and then I ran a ratchet strap over top of the surfboard, and through the windows, and strapped that sucker tight.
I drove a total of 4000 miles with it on top of my car like that, mostly speeding too. No problems.
But, I would just buy a rack, Toole has one for $100 that is awsome, and you can go buy a set up that is made for it, that is pretty much the same idea that I did with the noodles for about $40, but if you are that strapped for cash, buy some noodles and strap the board down with ratchet straps. The noodles let you tighten the board down really tight, and the board pushes the noodles down, so the noodles are pushing back up against the board, if you just strapped down the board without some type of foam or something, I don't know if it might break loose if you are driving fast.