Key being a tight fit (close) fit around the anvil..
A flat bottomed anvil this is a great way.. Hay Budden, Trenton and a few others used a hollow base... In this case a plate of 1/8" or more under the anvil would be needed..
I've tried the sand method as well with the hay budden with sand and it lasted 5 minutes as the sand just displaced filling the cavity.. I then tried to fill it with sand, pebble mix mounding it up in the center and sliding the anvil on top so the cavity was filled.. This worked for about a week than I noticed each time I hit the anvil there was a faint cloud of dust.. I pulled the anvil off to find the pebbles had started to break down rock crusher style..
Then I tried the plate over the sand and this worked well but found with how aggressive I was with the anvil when working that in short order I had the hole wallowed (worn wood).. and when working at the heel or horn the anvil would move around..
I then came up with an idea based partially with an article in the Blacksmith/wheelwright that showed a bar wrapped around the waist and bolted to a angle iron stand..
I took this and moved it a very large leap forwards (for me at the time) with 1/2" round wrapped around the waist from 2 sides threaded on the ends.. I then bored a hole completely through the center of the stump and burned a 1.25" square bar through the center.. This then had 4 holes drilled though at either side of the stump with enough room for the nuts to be tightened.. This worked well enough but the 1/2" bars started to bend at the ends where they were threaded through the square solid since the holes were 90D and the bars were at a narrower angle..
I then came up with this example and it worked flawlessly for years and years.. the only tightening that ever needed to be done was 15 years after the install, the though bolts needed to be tightened as the stump had shrank some..
I moved up to 5/8" round for all round bars and the 2 main round bars went through the stump.. The straight bars were let into the wood with notches at the same angle as the stump.. I then bent the ends to be the correct angle for the tie rods would pull straight.. The straight rectangular bars were 1"X 2.5 IIRC.. punched hot for the 5/8" bolts..
Anyhow it worked flawless till the stump lived outside for 5 years and if rotted away.. You can see it as removed from the rotten stump..