When you do the math, you quickly realize the effects of pressure! If you are going to stay above 5 meters, then you will be safe if the bag has 33% more air in it than the volume of air within the camera. If you are above 10m, then you need 100% more and so on. Estimating the air in the camera may be difficult, but it is certainly less than the volume of the camera itself.
The beauty of the compensated bag is that it is constant volume and the device can go to pretty impressive depths. I was designing a rigid case a while back and realized that the net pressure on the housing at 100' is 45psi. At that point, I decided to buy a case (this particular case has a 24sq" back, so it is opposing a force of over 1000 lbs).
At any rate, as long as the camera is relatively small, the changing volume of the bag won't cause enough of a change in buoyancy to bother you, so an oversized bag is probably just fine for the depths that you are talking about.
One thing that you might want to check out is whether the camera has any enclosed air-spaces in it, in which case the increase in ambient air-pressure might cause problems (e.g. the LCD display might have problems). This consideration would apply whether you are using the "oversized-bag" technique or the regulator technique.
Good luck and if you build this thing, please post pictures (of the device and the stuff that you photograph with it)!