What i would do is trim it so it sits under the tranny bolt area and it should hold itself in place , then where it extends past the frame , bend it slightly down (propane torch or heat gun) and shape that end like a funnel , so the oil will drain into a pan easily when the bike is on the kickstand (or leaning against a bike stand with the footpeg) , if you trim and bend everything right , you should be able to put it in place , then undo the bolt and remove it then replace it afterwards , without moving the tray or making a mess....
What you might want to try is to make a cardboard mock up of the tray , then try shaping/trimming it , before , you start cutting on the actual tray , this way you know what fits/works before trimming/bending something you can't change back
