I had a situation in my game where i simply had to have portals. These were very quick to build but overall I was happy with how they came out.
Use Google to find a portal looking picture that you are happy with. Print 2 copies of your picture on to the thickest paper you can get for your home printer. I use 200 g/m2. This is sturdy strong paper that had many uses in my craft.
Cut them out and glue them back to back.
You will need a base of some kind. You could use a spare one, make one out of Hirst floor pieces or perhaps just use a piece of strong cardboard. I managed to get my hands on Bones 1 and Bones 2 so I have a bag of model bases at my disposal.
Glue a Hirst Arts block to the your base. Glue the bottom of your portal picture to the piece and glue another Hirst Arts block to the other side. This will hold the portal in place. In this picture I have used the angled fieldstone piece from Fieldstone Accessories Mold #71.
Paint as you desire and get in on your table! :)