Don't know how big the piece of plywood is that you have cut but, if you still have some of the plywood left over, you might use the original cover as a pattern and cut an additional piece of plywood that will fit into the sink. Then using a good glue, glue it to the bottom of the piece of the plywood that you have cut. This will allow the bottom piece to fit into the sink and keep the top portion you are doing work on from slipping around.
Dave and Karen -- '02 Ultimate Advantage 36 C, 350 HP Cummins, Allison 3060 Tranny
'13 Ford Edge, InvisiBrake