So our neighbor's cat has been visiting us for a while, so I decided to build this ramp so that she could more easily get into and out of our yard. I'm not much of a woodworker, but was happy with how this turned out.
It's made almost entirely from cedar fence posts. Nearly 18 feet long (3 fence posts end to end with the dog ear tip cut off). Ripped cedar planks into 3 for a frame/support to prevent flexing. Posts are pine 2x4, and go 18 inches into the ground, which was the majority of the effort, since our ground is like cement...
Letting a cat run free makes it kill countless birds, frogs, insects etc. It also endangers the cat itself. Cats should be kept indoor, or brought outside on leashes (though there isn't a "cat proof" leash they cannot get out of sadly, so it's risky too). The only "moral" outside cat is a barn cat, and only slightly.