understanding the 'extrude'?

[micheus]

I don't think there's a way to use extrude to fill up the 'hole' and make the mesh enclosed again by the way...

As in any other application, some conceptual issues needs to be assimilated.
If you understood the Wings' Hole concept, and checked the options available for that, you should to know that if you want it enclosed again you will need to select a boundary face and use the RMB option - for remove the adjacent holes. Simple like that.

We do not have a face with two sides on Wings3d. If we want an interior for the object, then we'll need to use intrude, or if no "hole" can exists, we can use a copy of it inverted and then group them.

As in all application we need to learn how to use the available tolls. We always would like to get the best of all worlds, but we can't.

PC, you can get your hole material easily, just create it as it was already suggested here in the forum. I still prefer the WYSIWYG concept.