If you have some fabrication skills and access to a welder, you could make a cart just for your 'Hopper that would rigidly bolt or clamp onto the outside edges of the mower's steering arm. Mount swiveling castor wheels on the rear of the cart, just like your mower has, and it would spin with your mower: sort of a "zero-turn" lawn wagon. You could go the extra mile and make it a dump cart, too, if you wanted more versatility; you'd just have to mount the castor wheels directly under the tub's pivot point. Either way, you'd need to be sure you have enough space for the whole thing when manuvering around your property. Since the mower & cart would rotate as one while turning, it makes for an extra-long pivot - you wouldn't want to accidentally slam it into anything (or anyone!) and damage your new wagon or your mower!
Once I get a polebarn put up next year I was thinking of turning this idea into reality. I have a nice poly tub dump cart, but it's mainly for hunting. I suppose I could adapt it with some sort of sub-frame bracing to work with the 'Hopper. For me, half the fun will be to see if I can build the thing and actually get it to look & perform the way I have it in my head. Good luck, whatever you decide!
