Working with transparent footage?

[Ampix0]

I have created a few 4K transitions and my Intro for my youtube video. Previously I directly imported the AE file, which made for incredibly slow playback making it hard to ensure the video is lined up properly. I wanted to export these files in 4K and 1080p for proxies. I'm having some issues exporting video with the alpha channel though. Various tutorials just seemed to fail. Lossless AVI I believe did work, but the file size is 2GB for 2 seconds of a transition.

Should I just continue working with the AE file? Does anyone have a better tutorial for exporting video with alpha?

[JPT7505]

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