I see! That makes sense now that you described the issue with the land. The others have posted great ideas about providing bottom coverage.
I do use a chicken tractor similar to that for short outings so my chickens can graze without fear of a hawk or bobcat grabbing them but I don't let them into the tractor for a few hours, it's not their permanent home. The idea of a removable predator skirt would work in your case. Maybe instead of having it full attached, you could have a "line" of hardware cloth that could be anchored in with stakes or those big garden staples.
I do use a chicken tractor similar to that for short outings so my chickens can graze without fear of a hawk or bobcat grabbing them but I don't let them into the tractor for a few hours, it's not their permanent home. The idea of a removable predator skirt would work in your case. Maybe instead of having it full attached, you could have a "line" of hardware cloth that could be anchored in with stakes or those big garden staples.