Generate 360 cubemap, save with compression?

Hi all

I'm captuing a 360 picture of my scene, to use as a skybox in a loading screen (this is a webGL project), and adapted from https://medium.com/acrossthegalaxy/making-skyboxes-directly-in-unity-7250fe90c632
Code:

Code (CSharp):

1. using UnityEngine;
2. using UnityEditor;
3.  
4. public class SkyboxCamera : MonoBehaviour
5. {
6.  
7. #if UNITY_EDITOR
8.     const int TEXTURE_SIZE = 1024;
9.     void Update()
10.     {
11.         if (Input.anyKey)
12.         {
13.             Capture();
14.         }
15.     }
16.     void Capture()
17.     {
18.         Debug.Log("Going to render cubemap");
19.         Cubemap cubemap = new Cubemap(TEXTURE_SIZE, TextureFormat.RGB24, false);
20.         cubemap.name = "Skybox";
21.         Camera camera = GetComponent<Camera>();
22.         camera.RenderToCubemap(cubemap);
23.         EditorUtility.CompressCubemapTexture(cubemap, TextureFormat.DXT5, 0);
24.          AssetDatabase.CreateAsset(
25.           cubemap,
26.           "Assets/Materials/Sky/Skybox.cubemap"
27.         );
28.         Debug.Log("CUBEMAP RENDERED AND SAVED!");
29.     }
30. #endif
31. }

The problem is that the cubemap hardly supports compression and thus is quite large. 6MB for a 1024 cubemap.
One of the few compressions that I found worked was DXT5, which is funny because cubemap is supposed to not support any compression.
How would I save it as a modern skybox with compression? I don't need HDR or anything fancy