For a great mobile solution and one I use in the studio a lot too, get a big 5-in-1 reflector. These are great as a wrinkle-free background that you can fold up and take with you. the big square ones are so firm that they will stand without aid saving you another light stand to drag with you. The amount of uses for these are endless.
They are called "5-in-1 reflector" as there is: silver reflector and black on the outer cover. Turn the cover inside-out and you get warm gold and white, take the cover off and you have a diffuser panel as seen on the below photos.
Show uncropped below for sake of explanation
Notice the shadow when the subject is too close to the background !
It is also great as a massive diffuser for your speed-lights or studio strobe when put close to the model or subject and the light source behind.
And if you combine the above two techniques you can get a pure white background once you light your model separately in front of the reflector used as backlit background.