On my last 6-week tour I used old 700c inner tubes to lash my tent and bag to the rack top. One for the tent and one for a drybag containing bag and small sleeping mat with a non-inflatable sitmat rolled up longways on the rack.
I used about 4 wraps to prevent pressure points and tied the ends (where I cut the valve off). It proved very secure even over rough tracks and the tubes lasted the whole journey.