Most breweries I see have their big barrels on steel stackable shelves.
I would use some heftier lumber and make a cradle with at least 2" thickness by 8" height on the front and back faces. Match the contour of the barrel with the front face and rear face cuts. Look at some of the small barrel holders and scale it up.
I went with 2x6's, big bolts, and lots of supports. I didn't want to post back until I knew how it would hold up. So far so good. It's been full of Bier de Garde for a few months now. No leaks and no creaks (or Krieks).