I bought a 12" x 12" piece of 28 ga. galvanized sheet metal at Home Depot for about 8 bucks. Threw it on top of a bookcase and it works fine inside with an inexpensive mag mount antenna by my window.
Amazing isn't it, how all of those VHF handhelds have been transmitting and receiving just fine for 40 years, with no 19 inch chunk of metal under them. My TH-F6 hears two repeaters 90 miles away on it's standard "rubber duckie".