I can talk you into buying a WM sleeping bag.
Just do it.
I bought a 0 F Antelope Super Dryloft Regular in 1994. Last year I bought a 20 F Alpinlite Large. Both are superb. True to their temperature ratings, maybe below with warm layers and a sturdy shelter. The hood, collar, draft tube and foot box in a WM bag are worth the price of admission. Buy quality once.
Wayne