I am not an underquilt user, but from what I've read, the general goal is to hang an insulative layer that presses gently against the bottom of the hammock so as not to lose any loft, in a manner that all edges are pressed against the hammock to prevent drafts. If you search the forum, I think you will find a few different ways to do this.
I had suggested the catenary cut because I know it works well on tarps to keep a taut edge. I don't know that I've ever heard of anyone applying that concept to an underquilt. Perhaps some of the underquilt clan have an opinion on this?