To bevel the belly in a way, that you get a wider belly than the backing is, will be, from my experience, the savest design.Rhen you can take a backing from a smaller bamboo pole (8").
One exception, possible for backings from thick poles, may be to cut down the backing so that there is no visible edge of the bamboo when the bow is finished. From the forces point of view, this can be seen as the same as describes before.
And if you have a strong wood for the belly as it is with good osage and a backing from a thick bamboo pole (10"), you can also go with a normal design (no beveled sides) and a backing approx or a little more than 1/20" thick at the side.
The oval cross section I do not like so much for this bows from performance point of view, as it implies is some "dead mass" in the limbs side which may slow down the bow.
Thats just my impression from sonme, but not tooo much bows.