Go to the Armstrong site and take a look at their 'wave' product, it will give you some ideas.
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I would not worry too much about it. Yes, it will result in a standing wave that can be calculated by the distance between the vertical surfaces and the front and rear walls. But that is probably a small factor next to the standing waves from the walls, ceiling and floors themselves if your room is rectangular. the reflections off of the vertical surface facing the speakers are gong to bounce up into the ceiling and then back towards the speakers. By the time that wavefront makes its way back to you it will be pretty low in level and far enough out in time that it will just be in the reverberant field.
I think you are probably ok. If it concerns you, just add some fuzzy product up there, but I don't think it will make a very big difference.