I hate to be the bearer of bad news. But you are never going to regulate this room comfortably. Nature is working against you. The lower level will cool down before it gets cool in the upstairs room. If you set the stat way lower it will just freeze you out downstairs. You may want to try a window unit. Or even one of those PTAC units like in a motel room. I don't think adding a return or a booster fan, would give you any noticable diffrence for the amount of money you would spend having them installed. You could try one of the Sanyo or Mitsubishi mini splits, probally the most practacal solution since the space is fairly small. They are kind of exspensive, but work pretty well in situations like this. Or you could have a zoning system installed on your exsiting system assuming it is large enough to carry the extra load. They are kind of exspensive to install also, but they work well. I like Carrier's Comfort Zone 2. It will work on anything if installed correctly with the right suply dampers and a properly sized and balanced bypass damper.
duct tape and cable ties will fix anything, if that don't work add a 90-340.