I have done a little more on the 'mystery instrument' this weekend (it shouldn't be too hard to guess what it is!). The sides (and back) are cherry, which was quite nice to bend. I used to soak wood before bending it, but since working on the bowlback mandolin, I have discovered that this is not necessary (unless it's really thick).
The form is made from pieces of 1" plywood glued together. This is quite expensive to buy new, but I discovered a nearby social enterprise called Chiltern Wood Recycling, that sells ideal materials for this sort of use.