Parama Grave Digger shell...
ProLine Bug (long version, even if they say differently, idiots.. in fact most of the ProLine bodies are listed WRONG. Check the wheelbase of the Flux, use TowerHobbies who generally have the wheelbase listed for the ProLine shells, and use that as a reference)
But really any body for the Savage X will basically fit the Flux (see the HPI Savage X manual for a pretty complete listing of HPI bodies). The biggest issue is the battery boxes; bodies either will just barely fit over them or in some cases (the Bug above has a slight rub on the front part of the body) have a bit of rubbing. Either have to jack up the body (personally I hate that look) or what I've done is used electricians black tape on the inside to give some cushioning; thinking of maybe switching to Gorilla or something because sometimes the electricians tape wants to come upstuck.
Both the Param and ProLine are 0.06 lexan, most of the HPI bodies are 0.04 :( But I do have a Savage X GTO body I'm working up for the Flux too.
And of course there is the HPI GT2 shell!