Step 3: Make drill holder
Cut a hole big enough to snugly hold the collar of the drill. Screw this piece to the piece of board which is just a bit shorter than the drill body (base board).
If you have one, fit a long straight piece of bar, or dowel, in the drill chuck. This will soon show up any misalignment. Fit the drill collar into the holder you just made. Prop up the drill body with the handle vertical or horizontal so it is held with the long bar parallel to the bed and to the edge with the strip attached. Run the drill (slowly if it has speed control) to check for wobble, so you know to compensate for any.
Measure the height of the top and bottom surfaces of the drill body at the position the second piece of ply is going to be. Draw a vertical centre line down the piece. Mark the exact halfway point between your top and bottom lines and cut a round hole centred on it.
File the hole to fit the shape of the drill body, taking care not to go past the top and bottom limits you marked, and not to make it so the drill is twisted at all.
Fit the drill into the two boards and screw the back board to the base board.
Check the alignment again and secure the base board down to the bench. Re-check the alignment. Adjust any tilt using shims. Gentle application of a hammer to the base board seems to be the best remedy for horizontal misalignment.
I found it necessary to add a couple of wood bars to the top of the drill holder, to stop the drill collar being pushed out of its hole when pressing hard with the workpiece (photo to follow)