Tip them over, stack them from front to back in the case.
Have a couple fans in the front pushing air through the case - will be plenty fine cooling.
We have experimented with that setup but the 2U is not tall enough for this. So yes density is the name of the game but if you are using a 4U you really have not gained any density per rackspace over the 2U. Which we can get 10 units as shown with right angle power and usb cables.
It depends on your xtra rack spaces and preference. We can certainly do the "sideways double stack method, jut not in 2U.