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FWIW, I use one of these...
Topeak® Cycling Accessories — Products - MTX TrunkBag DXP:
...though I don't think it'll offer you any advantages to your existing racks. I, personally, wouldn't trust this or any other rack with anything more than a pocket camera. I'll concur with trenchmonkey, I can't see anything good coming out of having the internals of your DSLR gear constantly getting rattled around. That I were so inclined to take a DSLR along on a bike ride I'd probably just load up my Glass Taxi and wrap things inside a towel just for the extra protection!
Hope that helps!