I started out with 2x10 pressure treated 12" long and they cracked in the centre, tried the trex type, plastic composite and they cracked as well. I made some 12"x12" pads with 2 layers of 3/4 " pressure treated plywood that I coated with rubberized stone guard. They worked well but got stone marks and the bottom of the jacks wore the paint off, so I glued 3/4" rubber from a stall mat on the bottom and a thin piece of vinyl to the top. They are a little heavy but don't absorb water or get scraped up.
I have 4 of them if I need the height. For the rear jacks I have 4, 6x6 cedar blocks, 12", with vinyl top and bottom and sides coated with stone guard.
Married To The Navigator
2005 Red GMC Duramax 4X4 HD Crew Cab
4" Magnaflow, Banks Air Intake
Firestone Ride Rite Air Springs