MUV (Lizzie Lenard) has a video on her blog that shows her fmq on her old machine using plastic wrap to cover the bed of the machine. I have forgotten what she does about the feed dogs but it would probably be a good video to watch. Personally, I love the ingenuity you put forth to conjure up and manufacture a cover plate but from the looks of the photo, it appears that your quilt might get hung up on the raised edges of the cover. Maybe not, so guess you would have to try it first to see how you do.
Here is her video in case you want to watch it (maybe you have seen it already?):
https://www.youtube.com/watch?v=xkXJ...eZc5g1DU4tdM9A