I would use a glue block rather than a recess.
I would make the glue block first using any fairly dry hardwood about 2" thick.
I have one from walnut on a dedicated faceplate I've used for about 50 NE bowls
Lyle Jamieson has nice video on making a glueblock.
http://aawvideosource.org/php/link.php?t=video_detail&f=link&i=81
Then I would mount the platter on a worm screw in the chuck with a 1/2" deep hole
I would use a plywood spacer ( small square with a hole in the middle) so that the worm screw stick out a bit less than 1/2"
I would turn the bottom of the platter and make the mounting area for the glue block.
Lyle has a nice video on prepping the surface. This is same way to prep the surface for a faceplate as in the video for making the glueblock. Shallow concave ensures a good contact with the outer edge of the faceplate.
http://aawvideosource.org/php/link.php?t=video_detail&f=link&i=165
Mount the glue block as shown in the first video. One thing I do here that isn't clear in the video is to twist the glueblock a little as it makes contact. This helps spread out the glue.
Turn the top of the platter. Need to finish this before the wood moves and breaks the gluenjoint.
You have at least 2 hours.
Remove the glueblock with a flat chisel aimed at the glueline
Light taps in one or two spots will fracture the glue.
To return reverse chuck with the center point out of the live center and either cut a recess or use the glueblock.
videos can be found using the search feature at
aawvideosource.org Category- chucks¢ers Keyword-glue block