It's actually ground plane. Phil.
A 3-foot diameter piece of sheet aluminum will work, but there is a big problem with it becoming a wind catcher.
A better idea is to use an antenna actually designed for home station use. I would suggest a CushCraft ARX-2B for 2m only or an CushCraft AR270B, if you want a 2m/440 antenna. Both are far superior to your home-made J-pole and are good performers.
Whatever you decide on, you want it as high as you can get it.
Lon - W3LK