I've done it before, but on an SUV with Yakima bars that were spaced apart. Outback's racks aren't that spread apart, so like 4wheeldog said, you may want to tie down the front, maybe to the front tow hook. (is there one?)
A 4 x 8 sheet of plywood will (just) lay flat in the middle of a bed of a regular 8-foot bed pickup, between the wheel wells. My pickup only has a 6 foot bed, so the plywood only fits if I lower the tailgate. A little visualization puts the Outback in a different category. Some arrangement involving the roof rack may be possible, if it's done securely.
Sometimes I miss my '83' two door hatchback Subi. It would swallow a 4x8 sheet with the backseat down and the hatch tied closed. The sheet would stick out but it could travel cross town without problems.
In addition to the width of the floor between wheel wells being only 43", the width of the gate opening is also too narrow, at least without tilting the plywood sheet. It MIGHT work to have the sheets on an angle, propped up on one side, but you'd still have a couple of feet hanging out the open hatch.
On the plus side, it IS possible to move a few pieces of 8' tall lumber (like 2x4's or 2x6's), by laying the board ends on top of the center console arm rest.
Probably the largest single item I've had entirely inside of my OB is a Weber Genesis grill, unassembled and in its original packing box.
Actually, I don't think I could fit a 4' x 8' piece of plywood entirely inside my old Ford Expedition. The width was OK for the sheet to lay flat, but I think the length of the cargo area was only around 7' or so, so the hatch still had to be kept open. On the other hand, my even older Plymouth Voyager minivan would take the full plywood sheet, flat on the floor, with the gate closed. A real man's vehicle, no doubt....