Fill in mesh?

link textIf you haven't solved your problem yet.

Try using this: Triangulator

It worked for me with a non-convex shape.

Update

The link is off so I'm uploading the code I took from the unity doc.

Explanation:

So when Meshes are drawn, the mesh is constructed out of triangles which are only visible from one side. If you want to have the inside drawn aswell you essentially need to reverse all the triangles so that they are drawn from the other perspective then add the original perspective. This doubles the size of all your meshes. Below is a script I just wrote, not tested, that should draw the inside and outside on the start of the object lifespan. Just add the script to any in-game transform.

Solution:

     using UnityEngine;
     using Linq; // Add this <------------------------------------
     using System.Collections;
     
     public class FlipInsideOut: MonoBehaviour {

      void Start () {
         // Get the mesh
         Mesh mesh = GetComponent<MeshFilter>().mesh;
         // Get the both the inside and outside triangles
         int[] inside = mesh.triangles.Reverse().ToArray();
         int[] outside = mesh.triangles.ToArray();
 
        // Combine them into a new mesh array
         int[] combined = new int[inside.Length + outside.Length];
     
         Array.Copy(inside, combined, inside.Length);
         Array.Copy(outside, 0, combined, inside.Length, outside.Length);
 
        // Set the mesh array
         mesh.triangles = combined;
      }