Fit the other housing dado joints together to make up the carcass of the shelf. Use a tape measure to check the distance between the sides at the top, bottom and middle of your piece. If it is too wide in the middle you can clamp it until it is the same as the top and bottom before you mark it.
Place the middle shelf in place and make a knife nick to mark the length needed and then square it all the way around keeping the bruising from the knife bevel on the waste side of the wood.
Create a step down using the chisel, flick away the waste material, this will stop the edges from fraying when you cross cut.
If, once you have sawed the shelf to length, you haven’t reached your knife wall, use the plane in a circular motion until you are down to depth. Make sure that you don’t go all the way to the other edge as this risks the corners breaking off and splitting. Instead, change the position of your wood in the vise and come from the other side.
Test to see if the shelf fits into the frame, use the heel of your hand to lightly tap on the back of the shelf. As these are stopped housing dados, the shelf won’t reach the front of the unit. To ensure it sits flush to the front of the unit, you will need to take out a notch on either side of the middle shelf.
Clamp the middle shelf to make sure the joints are seated fully and mark where the notches need to be with a knife mark, square this mark all the way around.
Set the gauge to the distance of the gap from the recess to the edge, use this to mark out the notch across the grain and down either side.
Create a step down using a chisel, flick away the waste material. Saw down one edge of the notch using a tenon saw. An alternative method would be to split cut down the grain.
Pare cut across the grain into the corners to remove any fuzzy bits.