I'm guessing you are taking a machine quilting class. I don't own a Janome, but my suggestion would be if there is a darning foot that is made from clear plastic that would be the one I would choose. Some darning feet are all metal, which is fine, but the clear ones permit you to see the entire work area.
As for the tray, I purchased a tray for my Viking. It has a cut out for your machine, and it is snug to your machine giving you a large work space. I did a quick search to show you a picture, this is not the actual one you will need, call a dealer to see if there is one for your machine.
http://www.allbrands.com/products/ab...ogle&ovtac=CMP