ideas for cheap or free insulating under our cabin

our cabin floor sits up a foot or 2 off the ground in te front, alotta frigid cold air gets under there, i had a bottle of water sittin on the floor..and te bottom of it frozse!!!! its lil chhilly in here but not that cold, but the floor aparently isi know its only 1 of many weaterproofing projects we need to take care of, but just wondering if anyone has idea for what we can insulate under tere with thats cheap or free (recycled) we do have a sea of 2 liter mountain drew bottles out in the woods (11 years worth 1 or 2 a day thousands) if theres any way to use those it will save us alotta trouble loadin em in te bus to take to te dumpmaybe old matresses/?idealy wed still want room to get under tere to get at wiring and plumbingtis place is full of tweaker technology and needs alotta fixin up