No. I think it's not possible. You can't edit previews if the video is horizontal.
Found this:
> You can also tap Edit Preview to adjust how your IGTV video will be previewed in feed (you’ll only have this option if your video is 9:16. If 16:9, your video will show in full).
Apparently the support for horizontal videos still have some limitations.
Before trying anything update both Instagram and IGTV apps. They changed the way horizontal videos work recently.