I've been looking for a pattern and material for a similar project, something like the R1 pullover for use as a mid layer. The Patagonia R1 uses Polartec Powerdry which is a bit different than the Polarstretch..here's a good breakdown by BPL Staff Member Mike Martin -
"R1 is really Powerdry by Malden Mills (or used to be, anyway). Malden's Powerstretch is very similar, but has a higher lycra content to provide stretch. Both Powerdry and Powerstretch are available in several weights and finishes.
The Lycra in Powerstretch is fairly moisture absorbent. So, when saturated, Powerstretch takes longer to dry than comparable weight Powerdry. However, with small amounts of moisture, the absorbency of Powerstretch can provide a moisture buffering effect similar to wool, reducing "flash off" chilling from evaporative cooling."
One thing to keep in mind is that there are different compositions of the Powerstretch. Something like 49% Poly, 38% Nylon, 13% Spandex would be best suited for a tight fitting base layer application. At Mill Direct Textiles they have photos, a small description and material composition of most Polartec fabrics you can find online. Another cool thing about Mill Direct Textiles is that they have an E-Classroom containing a ton of information on sewing fleece.
I'm waiting on a sample of the Polartec 9407 from Outdoor Wilderness Fabrics. It's 88% Poly/12% Spandex, a composition more suited for a mid layer:
Have you found a pattern? I'm starting on a Thru-hiker Kinsman as we speak, thinking about adapting the pattern but it's pretty big. There are a few patterns that have caught my eye, both from Finland!:
1. Shelby Kaava Vuokatti Pullover
Looks fairly simply, and free! You do have to print out the pattern, tape it all together and cut it out..
2.Shelby Nallo Jacket This one looks realllly nice, I'm thinking simplify it and go a size smaller, so the fit is form fitting like the R1.