I do not use a mirror box because I didn't know about mirror boxes. I use a stand alone mirror (see photos August archives). I also use sliding wardrobe mirrors or a wall mirror I place on the floor. So long as I am able to place the mirror at right angles to my body and centered such that I can look in the mirror and see the image of one side of my body it works for me. I never look at the mirror image of a body part moving if it hurts to move that part. I may be mistaken but it seems to me to be counterproductive as I want the brain to register that I can move without pain.
David Butler, who wrote the book "Explain Pain" and who's group Noigroup sells mirror boxes and runs training world wide, emailed me saying that most research and clinical trials have been done with hands and feet. He added that there was every reason to trial it with a larger mirror as I am doing. David also said he thought that good therapists should also be using brain laterality techniques before and with mirroring as well.
If you have a physiotherapist who isn't familiar with mirror work they can access information and ask questions on a forum for therapists at the noigroup web site.
It costs me nothing. I do it anywhere there's a suitable mirror and it works for me. Seek professional advice before trying something new.