Top box, Givi, Shad or maybe something else?

[The Gecko_imp]

Remember that a long-arm rack like yours plus a big box and contents on top ... adds instability, in particular when riding with a pillion. The CB does already have a very limited allowed load, yet what matters here is not the total weight, but the fact that the front wheel gets lighter easily ... even too light. Your pillion (why else would you want a box for two helmets) will appreciate.
My Fehling rack says it cannot carry over 10lbs for that reason.
I know the CB ain't an airplane, but CoG and arm still matter.
So be careful.