if booleans work well in geomatic, it could be the plate minus cones.  booleans tend to produce alotta pointy trigons, but maybe poser can handle those now.

p.s.: if {plate - cone} doesn't work for chamfered holes, try {plate - tapered cylinder}.

I don't have booleans. That is the stumper. I would like to use them but if they produce an ugly mesh I'd rather not.

Cage has posted some work on a boolean mesh library ported to Poser Python. I haven't had time to look at it but he asked for help to convert it from recursive to iterative because as a recursive program it blows up the Python stack when you have a lot of polygons.

yes, cage's script works for chamfered holes.  generation of pointy trigons is less a problem when rendering metal shaders, but I dunno how to avoid 'em.  they also occur in carrara and maybe other boolean softwares.