How to make a Concave for a Budgie box?

I have been breeding budgies for 20 years and I have never used a concave in a budgie box but due to so many people asking about it I come up with my own design that is safer for you to make then the other designs that are out there.
My designs is made from 4mm brace ply that is cut in to four pieces that fit in the bottom of your boxes. You take three of the four pieces and drill a hole at lest 55mm from the end and side. Then using that hole as a guide and using a hole saw you cut a 95mm hole in the first piece, a 86mm hole in the second piece and a 67mm hole in the third piece. (See picture below)
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You then glue the piece with the 67mm hole to the piece with out a hole and then you glue the 86mm hole piece on the 67mm hole piece and then the 95mm hole piece onto the 86mm hole  piece.
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You can put a few small nails in to it to hold it while the glue sets or just put something with some weight on it.
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You can give it a bit of a sand it you want to remove any edges but the hen with chew it as she works the nest.
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