"I've used shallow bricks of sod cut from the rough area around camp." This clearly is not a good idea when camping anywhere other than your own private property. Cutting sod from any public or wilderness campsite is definitely going against the principles of "Leave No Trace" camping.
The whole idea of building an elaborate raft for a fire, though sentimental, nostalgic, or romantic, is quite a heavy-handed endeavor for any wilderness setting.
Canoe / Kayak Anchors
URCHIN Portable Anchor