Help with work in progress aliens model

Hey guys, I have been learning Blender as I go and to a point where I need more help. I have been reading online and have not had good success with two finishing items. I am attaching my model as reference

1.) At the very top of the model between the flaps I did the fill function so that I can 3D print this model. I want to solidify just this added fill, how can I do this?

2.) If you look into the model on the bottom is where I want a flat surface. Somehow with my work I messed that up so it is no longer flat, how to do I fix this to flatten so that it is parallel to base of model? This is so I can cut model open and place items in it.

Thank you for any help.alienegg only try 1.5.blend (1.88 MB)

The model already has thickness to it for 3D printing. But top flaps area meshes inside and outside are all tangled and intersecting. You need to clean that up. Hopefully you still kept outside geometry model to do it over.

For 3D printing inside geometry don’t need to be high resolution. You can build it manually to avoid complication created by fine detail at flaps area. Method is simple and quick. Box select and hide top half and hide it. Create the inside face out of cylinder. As for flap area work on section at a time by hiding other details.

how to do I fix this to flatten

Hit Z, box select all bottom vertexes, S , scale, in Z axis to zero. And don’t use Ngone as you have there. I think it needs to be a mesh.

Thank you for feedback. For top area where I added fill between flaps are you saying I need to redo? Would it turn out good if I did two fills at different vertices so that when exported and cleaned with netfab it could be seen as thickness?

I did the flattening comment given but when I try to fill more into the gap between flaps it just becomes very messy, what is best solution for this?

Problem is with how you added the fill between the flaps. The fill and thickness you added are created from single line of edge. Here is the cross section of fill geometry:

This make this geometry none-manifold. Its bad. Try to add thickness to fill continuous from both sides.