I currently am working in an exterior map (there will be some interior scenes but not till later) and am asking for any tips or tricks to portal big open areas so as to not get the "too many static models visible".

I have made my lod areas as model lite as I can without making it look sparse, but i still get the model error. My problem is mainly that i didn't take portals into my design right away and now I'm stuck with trying to fit it in sneakily or redesign everything.

I attached several images of my current layout.
The first one shows an overhead and the second one shows how it is currently separated.

I know someone who has been portalling longer than i have has some tricks of the trade that can help me. Or at the very least maybe someone can advise me as to how i can redesign the map so far.

Outdoor portal are always tricky as you tend to see a lot of open area and there is little to block the line of sight. What it comes down too is good planning, your map must be designed around the fact that is the player will be able to see a large area of the map, try not too over complicate things, such as have a ton of models of very complex terrain.

The only thing you can do to break line of sight outdoors is use buildings or tree lines, if you own COD:UO load up the mp_ponyri and have a fly about, that's a very good example of how tree lines can be used to break up the map.

Don't have too many portals in an open area, remember if the player can see them from any angle, the entire contents will be drawn. so having several all touching each other will not help and possible degrade performance rather than improve it because if you can see into one, you will most likely be able to see them all.

Other than that you done the right thing keeping things as detail (although terrain is automatically treated as detail on compile).