Summer cardi

My eye has yet again been caught by a summer cardigan, this time by Watershed. I like the way the fronts are designed to hang open, although I think I'd like the back a little longer -- probably to my natural waist.

The problem, though, is that I don't really have any appropriate aran weight yarn. I am swatching with La Laine, but it's really a light worsted and I can't imagine making it stretch to 4 st/in. But I have an enormous amount of yarn in the house & I don't really want to buy something new.

At the same time, I'm wondering why the designer and almost all projects on ravelry call for woolly yarns. I want to wear this in the heat of summer, over tank tops & dresses. Stores and restaurants are kept at meat locker temperatures around here (in summer; in winter they are overheated) so I need a little sweater but I am imagining a wool one feeling hairy and sticky. I'm thinking maybe Cotton Fleece, except that it's a worsted weight, too. (And of course, I don't own any.) What if I rewrite the pattern for 5 st/in? And make it longer? And change the yarn? And maybe the lace stitch? Hmmm.