I use different size carboys. I have 6 gallon, 5 gallon, 1 gallon and half gallon for all my secondary storage. It gets kind of strange looking at my wines. All the different size jugs sitting around. But it gets the jog done.
I have also thought about the marble, or like things. I saw bags of them at the dollar store. Can't beat getting them for cheap.
Primaries - Empty
Secondary - Several Hard Ciders, Traditional Bock
Kegged - Czech Pilsner
Bulk Aging - Brewtopia Barleywine