You can make the structure of the portal out of RKTs and cover in fondant or modeling chocolate and the inside of the portal, maybe you can do it out of isomalt that has been lightly colored. You could make a sphere out of the isomalt and as it starts to dry maybe use a toothpick or sharpened dowel to make the wavy look. this is what you will place on the RKTs and then cover the edges and began to decorate.
I think I would just layer fondant/gumpaste for the upright part with craft sticks inside to stick down into the cake and hold it upright. I would start with a solid circle and let it dry/harden completely flat, then layer the ring on top of that and finish with the symbols. I would insert the sticks between the circle and the ring.
Love Stargate btw; best sci-fi series ever!