Temp roof platform for 98 Imp Outback

Going to have to start putting the racks that came with my 98 Imprezza Outback to work. ( moving my mother out of her house). Was initially thinking of mounting 1/2 inch plywood to the the two racks, drill some holes to allow mutiplte load attach points. I know that the manual states no more than 100lbs on the rack. Anybody have any experience and or suggestions in putting something like this mount together, hardware used how to attach to racks etc. Thanks in advance............Doc