I’m in the process of making an underquilt. It’s not peapod style though. I chose to use synthetic (two layers of climashield combat) for mine primarily because this is my first UQ project and I thought it would be easier to work with. Pretty sure I was right about that. One advantage I found with synthetic was it helped me develop my differential cut. I actually made a form out of corrugated cardboard that matched the shape of my inner shell. I layered insulation on top of that and literally measured the length and width for the outer shell rather than calculate it. It would be hard to do that with down.
That said, getting down to 20 deg F. will require more insulation that I used for mine, 3 layers of combat or the equivalent in my opinion. That’s considerable bulk, especially if you make it over sized like a pea pod. Down will definitely be warmer both per oz and per volume.
Either could work. Its just what you want out of it. You can always make a synthetic UQ first, sort of develope your design, then clone it using down. Pretty sure that’s what I will wind up doing eventually.
