Patch concrete upside down

I hired someone to create a hole in the concrete block wall for a window. However instead of using a saw he used a sledge hammer and created a larger hole then necessary with a very uneven edge.
Now I need to patch the concrete block with concrete cement to the desired rough-in opening for the window. I am using two pieces of wood on each side to help me fill in the cement along a nice even edge. However along the top edge, I need to fill in the block wall holes with solid concrete but it's upside down so how can I do it? I imagine anything I put up will fall down right the way. Any tricks?
Thanks,
MC
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Since you've already got a rough edge that needs to be repaired, could you open up one side of the upper block a little more and insert your concrete from there?
In other words, build your form so that it covers one side and the bottom of the block. Brace the bottom board from below. Then fill the void from the side you opened up, using a stiff mix, until the void is full. Then attach the last side of the form to hold it all in.
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