Aether-like Portal Help

I am making a mod that adds a new dimension, and to get to the dimension, you have to build a frame of Obsidian, but instead of lighting it with a Flint and steel, you place a Lava Bucket, which is basically the same as the Aether's Glowstone and Water Bucket Portal.

I have tried many things to get this to work, but every time, the lava just flows into the portal and breaks it.

I want to use the Lava Bucket from Vanilla minecraft, not a custom Portal Igniter that looks like a Lava Bucket.

Any help would be greatly appreciated!