For two nights forget a liner. Your bag will stay dry enough.

In winter, on one week or longer trips, the only type of liner I'd carry is a waterproof, aluminized ripstop liner to keep perspiration from gradually wetting my bag insulation day after day. I got my material from Seattle Fabrics. Or you can buy one for $75. to $100. !!

In spring and fall I use my fantastic WM Megalite down bag & very light polyester long johns to help keep the bag clean, not really for warmth.

Eric