If you want to fill the shape I would do this:
- Duplicate the mesh without holes and place it at the exact same place as the mesh with holes
- Go into edit mode, select all faces and perform a scale along normals (Alt + S)
Now you have an inner surface. I prefer this over the method you suggested because if you fill the hole again with grid fill it won’t retain the curvature of the original shape. An alternative option would be that after the knife cut you don’t delete the inner faces but instead you extrude them. This would give you a similar effect but I think my suggested method is a little more flexible.
To answer your question on quads. No, you don’t need quad topology for a static object. It just needs to look good and shade well.
With subd modelling you can think of two paths that overlap but are not entirely the same:
- Subd model for animation where the subd modifier remains live for performance reasons
Deforming a lower resolution mesh is fast but looks crap. Using a live subd modifier gives you a way to animate “low res” but render in high res. This is the most demanding when it comes to your topology. Not only does it have to look good with subd on BUT it ALSO has to look good when it deforms which brings in extra requirements.
When people think strictly about subd topology rules they are thinking of this category most of the time.
- Subd model for static models where the subd modifier does not have to remain live although it is often useful to do so because it makes it easy to make changes later on.
There is a lot of overlap between the topology rules fore these two categories however there are things you can do with a static mesh that you can’t for an animated one. For example you can make use of polygons and poles > 4 edges when you KNOW a certain surface is 100% flat. This results in you having the option to terminate edgeloops on flat surfaces without shading errors. This is something you obviously can’t do for a deforming mesh since the surface will go from flat to curved and shading errors will occur.
You can also use subd modelling as an intermediary step to get a base mesh that you will then alter by destructive modeling methods as we did here. If you look at the contour of the holes we cut you will see lots of polygons that are not quads. This is fine as long as the mesh shades well (and it does because there is enough overal resolution to the mesh).
With that said its important to note that a lot of the general rules of topology don’t just apply to deforming meshes. If you want the subd modifier to subdivide and smooth your mesh nice and evenly the best way is still to have mostly quads and really nice evenly spaced edges. Even when you apply the modifier after the result will be nicer.
So… big wall of text but hopefull it gives you some context. Glad to hear this helped you!