Just about every neophyte amateur is looking for a way to circumvent drilling a hole and mounting their antenna correctly. It's the old conundrum; buy a decent antenna, and then mount it in the least efficient manner.
Yes, a flat plate will work, and in fact it will work to some degree if you do nothing in the way of a ground plane. But it isn't the radiation pattern that's skewed or reduced, it's the effective radiated power. Ground loss is just that, loss!