Open up the Hoth Siege .mapfile. You will notice above the map a small box with a portal_sky entity in it. You will also note a terrain model inside it. Just copy the whole thing and save it as a prefab under edit. Now load your Hoth map up and import that badboy in there. If you need help with a skyportal go here:
But this portal is already set up nice for ANY HOTH map.