i don't understand what you attach to the body, the battery?are there a model with screwing option then?
best solution for me would be a couple of extra batteries, as long as i can buy them 3rd party to save som emoney. Otherwise a powerbank, a slim one could do the job without adding lots of weight/bulky and at really reasonbale price. I don't remember where i read about dummy batteries to be not really safe in fx30 though