A tray isn't absolutely necessary if you can't readily find one and don't want to take the time to build one.
I just placed a scrap piece of plywood in the bottom of the tank to provide a flat surface & used an elbow fitting on the inside of the lid so the air doesn't blow directly down onto the resin. I don't do a lot of casting, so I just do one mold at a time and set it directly onto the bottom board. I'll take the time to build a tray if I ever want to do more than one mold at a time.