Hi, I am currently stuck... What would be the most effective, efficient way to fill up this hole.

Hi Erik,

At a guess, a two rail sweep of the two short end curves (from the edges.)
But for more help from forum members, please post a .3dm.
- Brian

Hi erik, are those long edges directly straight across from each other? If so then I'd probably either Extrude one side so that it touched the other and then use Edit > Join to join it to the main piece. Or also instead of extrude you can do Loft between pairs.

- Michael

Hi Erik, a friend of mine had the same problem once and what Michael suggested you worked fine in his case. I hope you'll have it figured out!