I'd get a deep bowl - such as a pyrex pudding basin type bowl, build up layers of tin foil around it and press is as hard as you can against the bowl, keep doing this until you have quite a few layers. Take the pudding basin out of the foil and this should create a bowl of foil ! Your coins should fit in fine and float like a good'un ! It's all about water displacement - ensuring it will float. An even, balanced shape which also goes beneath the surface of the water (to create displacment) will do the trick the wider it is the more foil you might need to create a rigid structure.
Good Luck