Very creative. I like it! Just a thought....you might want to tie 8 or 10 two liter bottles to the top of it with stainless steel wire to keep it upright on it's way down and after it has landed. Start out with too many so that it floats, and then cut the bottles off one at a time until the weight of the concrete pulls it down.
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Jesus is Lord! John 3:16 For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life.