I made a small cabinet that had a piece of plywood on top of it as part of a display platform for our Christmas tree. The plywood fastened to the top of the cabinet with screws. (1st image.)
I drilled holes through the plywood on top, down into the cabinet, then put threaded inserts into the holes in the cabinets. You can see the holes in the cabinet (which is also plywood). These holes do not go all the way through the cabinet top and, notice, one is partially in the side of the cabinet, so I do not want to run that all the way through.
Now that Christmas is over, we've realized this cabinet is rather handy and I want to cut another piece of plywood that will fit over this cabinet and extend back, behind the cabinet, to create a display platform for some small items in my wife's antique collection.
I'd like to line the plywood for the new platform over top of the cabinet, just like before, and be able to fasten it in with screws, like I did with the plywood for the platform for the Christmas tree. Of course, to do that, I need to drill holes in the new plywood that line up with the existing screw holes in the cabinet. The problem is aligning them and marking the locations when the two parts are joined together and I can't get to the screw holes in the cabinet.
What is the best way to do this? I can measure them, but I would think there's a better way to do this. Is there something I can put in the existing holes that will mark the bottom of the plywood for me? Or us just using a tape measure for distance the best way to make matching holes in a new piece of plywood?