If you have cross bars, it would be pretty easy. Just get some 2×4's- 4 to be exact. Two on the bottom and two on the top. Just to make them really stiff to not bend. Then ratchet straps then tight to your roof. Make sure you have a couple ratchet straps go from the front to the back to keep them from sliding forward and backward. If no crossbars, just get two 2×4's and put them on top, but you'd have to get creative with strapping it to the rails. But, you could use 2 ratchet straps to act as crossbars and tie everything securely to the straps and rails😂
Google "plywood on car roof" and you'll see many examples. Check your user's manual for the allowable load, in case you're thinking of adding more than a couple sheets. A sheet of 3/4 weighs around 70 lbs.
put a strap or rope(or two) running over the top from front to rear to prevent lifting and sliding forward when you stop.
I had an 89 GL hatchback that I once tied three 4x8 3/4 inch pressure treated sheets of plywood on top. It made driving very interesting.
If you want to securely carry stuff on the roof, Thule racks fastened to the roof rails work great. In the pic I am hauling 1x8x12’s on the roof. I’ve already brought 4x12 sheets of drywall home on the roof too. Had 2x4x14’s under the drywall for stiffness. I use the Thule racks several times a year, a worthwhile purchase for me, as is the Curt cargo rack that slides into the hitch. Great for smelly crab traps or bringing ethanol free gas back into Delaware for the lawn or small boat stuff, no tailgaters when you have several gas cans on the rack.