You might want to check scrap metal places for old aircraft landing strips. They are usually 10' long and 2' wide we use them all the time for enter and exiting sites that are real muddy. They were used by the armed forces for building temporary landing strips in remote places, at one time they were real easy to get.
They are fairly light because they are basically stamped metal with holes all over. They have tabs on one side that need to be cut off or bent over, otherwise you will cut your tires all up.
Good Luck