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To make the inside which you have highlighted you would need to divide the 10 " to get the pieces. I think you need a 5 piece finished and the corners 2 1/2 which will finish at 10 ". To get this size, cut a center square of 5 1/2 x 3. This will finish at 5 x 2 1/2 and add the corners cut at 3 x 3 to finish at 2 1/2 so you get your 10 inches. Cut the center squat at 5 1/2 and it will finish at 5 x 5. Cut the strips and corners as for the one side and repeat for the other 3 sides. You should come out with 10 1/2 unfinished which will finish at 10 inches square. draw it out on graph paper if you have some. Hope this helps if you are still wanting to make this block.