Hi Bravlin, so unless you had a really specific need to do it this way you would not want to approach your modeling steps so that you have to fill in an area like that.

Instead you generally want to have extended pieces that then all get booleaned or trimmed to common cutting surfaces, that's the best way for things to match up.

For filling it in though, one way would be to use the "Patch" command in Rhino which fits a bendy surface to a set of points sampled along curves. However, it is often quite finicky to get it to be accurate enough to join cleanly. Since this one isn't too bendy it could work though. In the future I also want to add in a similar surface fitting mechanism into MoI as well but there is not one like that currently.

A little later here I'll see about giving you some steps for reverse engineering the extrusion.

But again, you don't want to build things in such a way that you put yourself in this situation of needing to build a surface to a hole like that. It's better to work with solids and 2D curves from the beginning.

- Michael