As I stated

There are a number of problems with making a outdoor portal map, one of which is the fact that 3d skyboxes are not rendered when looking through a portal, which kind of makes the level look screwed up.

Of course you could design the level so that you don't need a 3d skybox, but that would most likely be a painful process, or the level would end up looking like crap.

Then don't use a 3d skybox?

Usage of skyboxes in Portal maps should be sparely, anyway. The whole essence of Portal is that claustrophobic inside of the Enrichment Center (not really, the whole essence is the portal gun of course, but the clinic enviroments add a lot to the gaming experience). But using a 2d-skybox is really not that hard. Simply apply the skybox-texture to the faces where you want the skybox to be. That will not mean the skybox is rendered on those faces, but you can actually "see" through them, and see the skybox "behind them".