I built the exact same size platform on our swimming hole and I used bigger chunks to anchor it. I wasn't worried about the kids moving it around really, more high winds. There's a good bit of surface area with those barrels for wind/water movement to grab a hold of. 2 years in and mine hasn't moved yet. I used the equivalent of probably 2 blocks per corner - total of 8. I used solid chunks of concrete poured flat with eyebolts through them - more surface area to dig into the bottom too.

Dale

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"When tempted to fight fire with fire, remember that the Fire Department usually uses water." - anonymous