I had plugged the countersunk holes for the wood screws, and also three screw holes which I had drilled on the wrong edge of one side piece.
So the plugs were trimmed with a Bonsai saw and a chisel, like this.
Here I am marking in a 10mm drain hole in the bottom rear edge of each side, using this marvellous Iris drawing compass which was a gift from my son. Thanks Nick!
The pivot and drain holes were drilled with the drill guide to make sure they were perpendicular, like this.