Do you think this will work ?
144-512 MHz, Ground Plane Kit Bracket
www.radioworld.ca
I wouldn't pay $114 for it. You can get them cheaper on Amazon or eBay. Basically, it's a ground plane. Will it work? Yes. Will it improve? Not sure, I have one, don't really use it. But if you put the Larson on it, and get it at a decent height, then I don't see why it wouldn't work for you.
The best option if it's available to you, or you are capable of this, is to get yourself a discone antenna and put it on a 30 ft. pole and you'll do just fine. Or put your Larson on a 30 ft. pole with the ground plane kit and get your signals better.