You could easily make one, or have one made if Janome doesn't have the one to fit your machine. Make a paper template of the shape of your machine's freearm. Take a piece of thin plywood and trace the shape onto it so that the right side of the plywood comes to the right side of your freearm area. Cut out the freearm area and sand the edges well. Paint with varnish or laquer to make the wood very smooth. For legs, cut pieces of dowel the right length to raise the table to equal the height of your sewing machine bed. Secure them by screwing from the top, through the plywood and into the dowel underneath. Use 5 feet, one in each corner and the 5th in the middle beside the freearm end of the machine.
If you don't want to varnish the wood, use peel and stick shelf paper/contact paper to cover the whole area.
If you want to attach it better to the actual machine, you can add an elastic strap to go around the machine and secure it with hook and loop tape. (Velcro)