Can´t help you with the 3d file but with another idea: I have an RS Props rubber foam DLT and it also came without a charging handle, this is the ingenius idea suggested to me by a friend: just take a flashlight and cut it up.
So I went to the DIY store, bought a cheap aluminum flashlight with a knurling that came close to the original charging handle and about the same diameter, took it apart, cut it in half, added a piece of wood with a self tapping screw, superglued it in place inside the handle, drilled a tiny hole in the rubber main body of the blaster, put some E6000 on the screw and screwed it in, I´m pretty happy with it, looks quite realistic if you spray it black and then sand some of the paint of again, like natural weathering.