I built one a couple of years ago, I cut down a big tree and used logs as backstops:
Dialing in new sights at 100 yards
I've got logs at 100, 200, 300 & 500 yards
I built a bench for left and right hand shooters, here's a photo without the top on it:
I would like to add a berm at 300 & 500 yards for fliers.