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Attach a board across the inside of the top of the legs. You want this to be something that you can drill and screw all the way through (like a 2"x 2") because you will be drilling through this to attach the top.
I then painted the base with Fusion Mineral paint in Putty. I used thick pallet wood pieces for the top. I sand the wood, lay the boards out and clamp both ends so the wood top doesn't have spaces in it.
Drill through the boards you attached to the inside of the legs into the underside of the island top.