My wakeboard does not fit in my trunk because i have subwoofers, and when it was in the backseat it was akward and unsafe to carry passengers. So i devised a way to hold my Wakeboard on the roof of my car. It hold's it securely, goes on and off easily, and can be made (in my case) with materials that i have around the house and shop. Good luck.
Step 1: Gathering Tools and Materials
-apropriate hardware (4 screws and nuts and some washers)
-and a saw (i used a table saw, but use what you got)
-wood, i used 1/2 inch plywood for the blocks and i think a 1x3 or something for the tops.
Step 2: Start to Assemble.
make two of these out of your wood. For me it helped to put them together first, before drilling. make the width of the small cubes close to the width of the your car roof rack. Make sure you leave the total length longer than the wakeboard or board that you plan to mount. After you have two of these cut out and what not, screw the two pieces together (but not through the center). This is just to hold everything together while you drill. Make sure your screws are not going to scratch anything, its a good idea to recess them.
Step 3: Drilling.
Drill through the pieces we made in step two first. Counter sink the top so that no screw protrudes and scratched the bottom of your board. Clamp the pieces to your current roof rack to hold them in place, then stick the drill bit through the holes that you just drilled to mark where you need to drill on your roof rack. (im not very good at explaning any of this, but if you can't figure this out from the diagrams that i made in sketchup then you shouldnt be reading this)
Step 4: Carpet
carpet the things. you can figure it out. use a staple gun (only on the sides).
Step 5: Attach to the Roof
stick your bolt through all the holes you drilled, use some washers here and there, and then use a nut on the bottom. its easy.
Step 6: Attach Board.
make two inner tube rubberbands to attach the board to the rack which should be solidly attached to your roof. in this case i used some rubber straps that movers use bacause i had them. as you can see in the second picture, i dont have one on the back, make sure you use two. And as you can see in the 3rd picture, my really old rubber straps stained the board a little. so be carefull, and it might be smart to cover them or something, youll figure it out.