Stick 'em on the ceiling if you can. I've tried the up-firing method (with up-firing modules, too) with little to no luck...or satisfaction. Though by no means a professional execution of the idea, I mounted some unused Monoprice satellites to the ceiling (picture 1), and then made little "cabinets" for them out of balsa wood, wrapped in white acoustically transparent fabric (picture 2). They slip right over the speakers on the ceiling - a purely cosmetic touch that still requires some cable masking. Point being, if you can get away with it, IMO it works best if you mount speakers to the ceiling, slightly forward of your MLP. And since the Atmos speakers are used purely for ambiance, you
can use some rinky-dink satellites, or whatever you have laying around. Not saying that
should be the solution...but it is one for amateur enthusiasts like me