Well, if this hay field doesn't belong to MyBoyPuck - a ditch is absolutely a bad idea unless permission is given.
I will tell you, properly made CC fences are not easy to make and take a skilled person who has experience in construction to help you out. Lots of cutting, sawing, drilling. I know, I've built fences and they take hours and knowledge.
Tire jumps are interesting and fun to jump, but difficult to make. You need to dig a somewhat ditch in the length you want the jump to be *one end to the other* so that the tires will beable to fit into it enough to remain supported and solid.
Then you want a long pole, to match the length of the ditch, or a tree stump small enough to work...whichever you want..... and then start placing the tires into the groove you dug, making sure the pole goes inside.
Then you want to hammer 2 posts, one on each side of the tire jump, and tie the pole that you strung through the middle of the tires with a strong and sturdy rope, with a proper CC knot. Then you want to take 2 more poles or trunks of a tree, and place them on each side of the tires to support the tires - sorta like you would with ground poles on each side of the stadium fence.
Eventually you get this:
I have many pictures of CC fences from the barn I used to Work and Train at - but they aren't simple to make. If you want to see more, let me know and I'd be happy to help you out.