Go to a friend and watch them fit your foot on their machine and use if possible. If not trip to a dealer and watch them use your foot on your machine if possible. The later may come with a bill the first may just be a homemade cake.
Good luck.
P.s. Try just using as a foot with two layers of fabric instead of your normal foot . If it works there keep making it thicker until it plays up. I've read on here people who use their foot all the time.