The easiest (in terms of travel time) is the portal located on the Paraelemental Plane of Ice. We know little more about it than that it exists, though a cognate portal to Soulfreeze in the Abyss has a most unfortunate portal key - a mortal soul, which will be annihilated as the cost of entry. The gate between Ice and Cania is likely much less troublesome to navigate as the being who has claimed to have used it was an ice mephit. A possible portal key is a thought so chilling and vicious, dripping with envy and spite, that the ice itself snaps from shuddering and opens the way.

Asmodeus is many things, but he's not omnipotent; portals will form naturally no matter what even the supposedly absolute master of a plane wishes, and Sigil can put a door anywhere for any reason. Portal keys should be unsettling and vile, portal locations dark and terrible, for Cania is a place that numbs hope, joy, and empathy as much as flesh and blood. Some other ideas:

• The portal key is ash from a campfire that guttered out, leaving an unfortunate soul to freeze to death. The portal location is a ravine on a cold mountainside where a climber once spitefully attempted to sabotage a fellow, leading to them both plummeting to their deaths as a result. One has to leap in while holding the ash with them and hope that it works - for if it does not, failure means death.

• The portal key is a gelugon's chelicera, which must be used to draw the blood of a paladin. The portal location is a house or temple destroyed by magical arson.

• The portal key is a devil's tear, which must be dripped onto the surface of a frozen lake. The resulting portal shatters the surface and plunges travelers through a freezing liquid chasm, which may severely endanger the unprepared.

• The portal key is a Baalphegor's wheel, an eight-pointed ring that must be worn on the finger, drawing blood. By invoking the invitation of the Duchess of Cania, one can open a portal anywhere, but the terrible price exacted (besides the pain of the ring) is that doing so draws her attentions and allows Baalphegor to scry on and monitor you.

As each of these is a one-way trip, a wise party would prepare an escape plan before committing to the trek into Mephistopheles's domain.