I bought one from this company:
http://sewingmates.com/index.php?mai...products_id=12

The good thing about it is when you buy a new machine it is adjustable and fits almost every machine. I love it.

I made my extension table from the blue foam board. I love it!

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)

My DH made me one of plexiglas. We marked fit & he cut and countersunk screws from the top to attach wood legs. Later we found adjustable legs. If you'll take your pattern to Lowe's or Home Depot, they might cut it for you. Mine cost about $18 and it's the heavy plexi. Labor was free at my house!