On a 2000 Outback, I would, if it was just a sheet or two of 3/4", 3 or 4 sheets of 1/2". If the distance was short and I could keep it under 35 mph or so. Probably cut a couple 48" lengths of 2x4 to use as extra crossbars and strap it down good. You can run those straps through the interior, too. I'm used to hauling my canoe around on top of the Outback though. I think the sticker on that roof rack says not to exceed 100 pounds or something like that.
I had a 1984 Subaru wagon when I started building the house, and I made a permanently installed 48" wide wood roof rack for it out of oak 2x4s, lag bolted up through the roof. They dinged me on trade-in for that a little bit. But it would haul 16 12 foot 2x12s. Drove funny though.