I agree, among all the swappables I know, FT Simple Soarer is the best candidate for winning the logest flight duration contest.
However I did achieve some pretty impressive results on my
ultralight FT Simple Storch, up to 34 minutes on 2200mAh 3S.
Also, you may want to look for flying wings with long, skinny wings; like the ones in this article:
One Sheet and Two Sheet No-Waste Flying Wings (especially the two sheet versions). Those are not swappable, but there are examples on how to convert a flying wing into swappable, you should not have troubles finding one.