I wear a half-wig 90% of the time. I put a plastic cap on first (the one from the BSS used for deep conditioning) then, I put a men's satin-like skull cap over it (it's like a doo-rag but it fits like a scull cap). I try to make sure that the plastic cap covers my nape area and that the skull cap holds it in place. During the day, I have to adujst the plastic underneath, to make sure my nape is protected. The skull cap does a pretty good job of covering the plastic cap and keeping it in place.
The benefit of wearing both caps is that I can baggy with the plastic one, and the skull cap acts as a barrier between my hair and the wig. HTH