Looks really good so far.
I would take the maple dovetail portion out and carefully take a few thou off the top before gluing. I just did a 2-piece (first time doing that) on a walnut stock for similar reasons. There was not enough figured-match wood for a one piece ptachbox lid, but I had plenty of plain walnut for the dovetail portion. No matter what when I glue, it ends up swelling a little or there's a immeasurable but noticeable glue thickness added and this lifts the lid off the buttstock annoyingly with a glued-up, two pice job. If you take a little off the dovetail, glue it up, and start to slide it in and it binds, you can work down the lid top or buttstock to get a perfect fit.
Next I would put the buttplate on, trace and saw out the dovetail notch in it, but also leave a little extra so you can hand fit things. Then when it's working all smooth and well fitted, you can trim the end off the lid and start fitting the buttpiece of brass.