Someone else can answer better than me, but what I recall is that the cup does fit tight in the back. There's a guy on another forum that fabricates & sells a square cup for the Tunze that fits back there easily. No idea what he changes but if you want the infomation PM me. It's just more money right?
What did you mod? Did you drill a hole in the top and run the air hose up higher to get easier access to an air valve?