World Portals - RenderTexture to match Viewport

I am trying to make world portals.

The only hitch I’m stuck on currently is that if my viewport size doesn’t match my RenderTexture size, it warps the visuals and they don’t line up - http://imgur.com/phHtBIq

If i resize my viewport it lines up properly - http://imgur.com/NdSZN68

Here’s my RenderTexture Material - http://imgur.com/N4VF6Xe

And Portal BluePrint - http://imgur.com/sXiHcsN

If there’s a way to resize RenderTextures at run-time that might fix it, or if there’s a way to scale the RT so it fits the portals regardless of what size the viewport is that would be better.

Thanks!