Or even better, an inflatable Dildo speciffically designed for the task. I bought one during my last trip to London. They had it at Regulation, but I'm sure you can find them at any well stocked S/M store. Me and my slave had plenty of fun with it. It was amazing how big it could get.
As far as safety is concearned. The weakest part of it is designed to be at the valve which means that if it would break for any reason, the air does not leak out inside the pussy, but is expelled without. I got it explained to me in the store that it's not possible to do any damage with it. They're designed to break first. Pain yes, but no damage.
There's two modells. One with a small dildo inside and one without. I got the one without but it can be a bit of a hasstle to insert when empty, so I'd recomend the one with a dildo inside.