If you have a way to anchor a plank perhaps in a woodworking vise this next solution may work for you. Even better if you can devise a way to rotate it. Any scrap of plank or ply for the back and some vertical battens should work. Be prepared for re-positioning the vertical battens quite a few times till you get the sweetest spot.
I have made single jaw vises on different occasions ( usually when in a hurry or lacking decent materials and so having to improvise. I always felt they didn’t grip the saw powerfully enough. That said, many competent filers do use single jaw so you may make it work. You can mount it on legs as the VERY competent filer David Baker has done below.