Luckily, there are many "thumb grips" available. I got mine on Amazon - theres dozen of places offering them online (link down the bottom of this article), so this isn't going to a specific product review, but more an accessory recommendation!
Its a simple little piece that is placed on the hot-shoe and tightened in place with an allen key. It makes the camera much more holdable, like the name suggests, gripped with your thumb! It's unobtrusive, and easily removed if the hot-shoe is needed to be used.
Buy on Amazon.co.uk (£5.50)