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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.)

Diagram of wood and cabinet

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.

Screw holes with inserts

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?

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    Cutting or tracing them onto paper or cardstock template might be more accurate than a tape measure. – Matthew Gauthier Jan 20 '18 at 0:26
  • That's a good idea, @MatthewGauthier. It seems like whenever I try to measure something like this, I'm always just a bit off. I was thinking of pieces of chalk in the holes or something I can slide in with a pin on the top that will give me a center. – Tango Jan 20 '18 at 1:21
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    Mark reference lines on the existing plywood top indicating the edges of the cabinet. Align your new top with those lines as desired and clamp . Drill through the existing holes. Voila – Shimon Rura Jan 20 '18 at 1:44
  • stick a thin layer of play-doh on the underside of the new top. then place the new top over the cabinet. ... etc. – jsotola Jan 20 '18 at 5:20
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If you were energetic, you'd take some bolts that fit the threaded inserts, cut the heads off, put a point on the remaining threaded rod (chuck the rod in a drill and spin it against a running grinder), screw the pointy threaded rod into the inserts and then press your new panel against the points.

You'll have an exact impression of where to drill the new panel.

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    Close enough to what I was going to mention so I gave it a +1. My variation was to use stubby round head screws sunk all the way into the inserts, and use a block of wood to protect the new plywood and rap the block with a hammer to make the round head screws leave a mark where the holes need to be. – Jack Jan 20 '18 at 4:33
  • That's an excellent idea - the only issue is that I've recently moved and a lot of tools are either at the old house or in storage - including a grinder. I'll see what I can find as a alternative to using a grinder. – Tango Jan 20 '18 at 4:35

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