I know I've asked this before, but I still haven't found any information.

How can one make a stable portal looking thing? I know there are several mods that managed itp; so does anyone know how?

Magic Hazard?

You mean something similar to this but with a teleport script attached?

https://www.youtube.com/watch?v=-K-OO6CpxbM

That would work, though I was hoping for something cirular. And I don't need a script, I just need the .nif, which I will use for a new door type.

You can make any shape or anim into a portal, I think.

For something more spherical look at "MAGINVIllusionLight01"

I think most of the maginv nif's are are round spheres. I think the one you'll probably want to use is maginvsummon.nif
However they do not have collison, so you cannot use the models as activators without reworking them in nifskope.
It tooks me days to get mine to work in game.

You can use the invisible activator placed in the center of the mag nif if you want the player to activate it or you could use a trigger box if you just want to teleport the player.

I'll be using the FXDA14PortalDoor, which can be found in Morvunskar (part of a Daedric quest), for my mod. Looks pretty good but does not have collision so you'll need to set up something for that.

I don't need collision, thanks for the infor guys. I had been trying to use SUmmonFXTargetActivator's nif, and it disappears shortly after appearing.

Yeah, some of them are effects that start and end quickly.
I don't know how, but I'm sure there is way to tweak the "timer" in Nifskope.
You'd have to open the model up and look for the setting.