Making a 3D Skybox (not to be confused with a 2D Sky Box) with Wall Worm is much simpler than in Hammer. Please read the more comprehensive series on the sky workflow.

1. Design your entire level at the same scale, including the geometry that belongs in the 3D Skybox. Design the sky at the same scale and location as if it were simply part of your map.
2. Select any brushes, models, entities and lights that belong in the 3D Skybox.
3. Click Wall Worm > Wall Worm Level Design > Set Selection as Skybox Item (or click the Skybox icon in the Level Design toolbar).

When you run the export, all items tagged as Skybox objects will be offset and rescaled around the Sky Camera.

• By default, when you open Anvil, the Sky Camera helper is placed in the scene at location [12288,0,0] . You can move that helper to any location where you'd want to place the 3D Skybox. Make sure that it is aligned to the grid!
• Brush geometry in the 3D Skybox still need to have the Brush Geometry tag.
• When using models in your sky, it is often best to use only WWMT Proxies. This helps alleviate offset problems that can happen if you are using models assigned to a WWMT helper. To keep the root model from exporting, select the WWMT helper and click Wall Worm > Wall Worm Level Design > Exclude Export of Model. Now only the proxies will export to VMF.
• The scale setting in the WWMT models in the sky should equate to the Sky Scale you are using. You currently set the Sky Scale of the scene in the Wall Worm Anvil UI. Unfortunately, at the moment, the scale in Anvil is expressed as the denominator of a fraction where as the WWMT UI expresses the scale as a decimal value. You can quickly set the correct scale in WWMT by clicking the 1/16 or 1/32 buttons... which would properly map to the VMF sky scale of 16 or 32 respectively.
• It may be easiest to manage if you place all of your 3D skybox geometry, models, entities into a separate layer. You may want to name it "Skybox".

More Settings

As long as you have a Entity Cache parsed from a FGD in your global settings, there should be all sky_camera settings available in the modify stack when you select the sky camera in the scene.