For those without bed posts you're in luck! I have a bed restraint harness that just sits under the mattress and wraps around. It's completely adjustable, and has four very soft cuffs on each corner. Just tuck it back under the mattress when it's not in use.
I've had the ABSOLUTE best sex ever using this thing. Restraints require trust, so if you're in a "relationship-building" phase do what I do and just start out with ankles restrained to hold her legs open but let her keep her arms free. She'll still get an ENORMOUS kick out of being unable to close her legs and knowing that you can do anything to her below the waist and she can do little but enjoy it.
Pic of my setup below.