RCi85's

Rumplestiltskin

I have 4 RCi85's in use for my back channels. Does anyone know what size in cubic feet is recommended for sealed box performance. I need to close them in because two of them have closed areas (about 2.8 cubic feet) and two have much larger areas/volume where they are really open. I notice a small difference in sound. I have ability to make them all the same and want to do so.
Thanks
Rumple