Domed nuts in the UK, I would place a sphere on a cylinder with the thread passing as far into the top as possible, and a 45 degree cone to finish the void - I have a module that finishes all blind threads with an internal point,  how a drill would finish the hole, which makes 3d printing simpler as there's no bridging required, ok, not necessary on small holes, but useful on large holes. Not surprisingly there is a standard form.. https://itafasteners.com/products-nuts-dome.php The first picture is a two part fabricated nut with a pressed dome retained with a rolled-over edge, rather than a fully machined nut. R.