For NFA items I've been using the MAF heat set serial number tags (https://maf-arms.com/product/heat-set-serial-number-tag/). I add the recess as a negative part in the slicer (there's a link to my STL file on MAF's product page). Then I drop the plate in with a drop of super-glue and use a soldering iron to melt over the posts to further lock it in.
If your FFL is just doing the engraving of a serial number you picked/registered, the MAF plates are cheaper/easier. Unfortunately, if your FFL is assigning a serial as part of a 4473 transfer, they probably won't give you the serial in advance. The heat-set approach may still be viable, but you'll have to source your own blank plate(s).