Open Polysurface issue

Hi there I’m trying to render a prison style toilet and am having trouble closing my open polysurface. I wanted to use the seat to cap it but Im running into so many problems. Ive trying all union commands as well as trying to close it with a surface but all of my efforts close the whole top but I need the bottom small hole to show (because its supposed to look like a toilet. Im trying to prepare this file for 3D printing. Any help would be greatly appreciated!

Dear @Ben22
hope the following file / info helps to repeat the “repair”

Benhelp_tp.3dm (3.5 MB)

make sure your edges / surfaces are precisly aligned before you mirrow them to make everything symmetrical.

focus on one half first:
_showEdges will show all naked edges
_setPt (in this case World x for the mirror-plane, and World z) will allow you to make stuff planar / straighten it
you can work with an additional (helping) surface (extrudeCrv) perpendicular to the mirror-plane.
then _matchSrf to this to get better continuity after mirroring


the result without the helping surface, just position continuity at the mirror plane is in the file.

kind regrads -tom