Well I made my first boom 21 block today. It is an extra for H4B. When I saw your sea turtle fabric I decided you needed one of this block, I made it a couple of booms back for an ocean quilt.
The technique has thick seams but as long as you aren't hand quilting shouldn't be a big problem.
Here it is before the "magic" of the folding edges.
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and here it is after the edges are folded and stitched.
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