Finding an elbow to work with 8 inch Sonotube would be a challenge. Other options would be 8 inch PVC pipe with an elbow which would probably be expensive. You could also try to redesign your box so a straight length of sonotube will work, or consider a slot port with the same surface area.
What did you end up using? If you haven't completed it yet, you could also mount the port at an angle so as to optimize the internal space of the sub while maintaining good clearance around the internal opening. Do you have a drawing of the sub with dimensions?
I wound up using 3 4" ports using pvc with 90 degree bends. The angle idea is a good one. I didn't thing of that.
I'm thinking of starting a new thread with pics of the build if anyone would be interested and if that's acceptable. Come to think of it I should start a small blog so anyone interested (and a few friends) could follow and see how the build is coming.
Is there any preferred format for such a thread? For example, do most people like to have all pictures shown inside the thread? Or if the builder has their own web page with pictures and documentation, is just a link to that ok? And I don't mean a generic thing like ImageShack or similar with one line titles to each picture. I mean a real web page made from scratch. Or a combination of the two?