I've been searching for a source of spare DFH-43 drop-in filter holders for this lens. I want to be able to quickly exchange different values of ND filter without having to unscrew/screw them in each time, but additional holders appear to be a rare commodity. It looks like Olympus did once sell them as accessories, at least in some territories, but haven't done so for some time. I'd be grateful for any input on this. Can anyone confirm if any of the commonly available filter holders from other manufacturers are compatible with the 300/2.8? I know that the Canon 52mm drop-in holder is too big, but perhaps one of the older Nikon, Sigma or Tamron holders might conceivably fit, or at least be modified to fit? Thanks!