A back-mounted
KFF comes in the MegaNobz kit, if you have some spare cash or were planning on fielding non-
KFF MegaNobz.
Other than that, it's a matter of rummaging through your bits box looking for electrode-ish thingies (which is pretty much how meks build their
KFFs anyway

) Tau and Necron bits are likely candidates. So long as it looks like a cobbled-together mysterious pile of electronics and not a weapon, you'll be fine.