Better Portals removes a lot of the restrictions to crafting portals in Minecraft. You can make them in any shape so long as the obsidian blocks connect somehow.
And even better is this mod removes the loading screen so you can see right into the Nether or End and transfer between worlds instantly!
Portals can even be made horizontally on the ground or in water!
You can even make really massive portals but with the early state of this mod you might run into a bit of a lag with chunks loading, but otherwise it works large or small!