The Milich adapter was designed for M8 and interferes with the corners of full frame.
You want this instead:
Leica 14473 E67 Filter Adaptr for 16 - 18 - 21mm F/4.0 14473
B&H doesn't seem to have them, so I got mine from Adorama. It screws on so that the two cut-outs in the picture end up at the top of the lens. The 67mm holder would block a chunk of the VF, but the cutouts let your external VFs or Frankenfinder 'see through' the top of the filter. If, however, the sun would shine through the cutouts onto the filter from behind, you put in the black ring to keep out stray light and flare.
Kirk
PS: Lightest Leica landscape kit: WATE +MATE + 24 f2.8 to fill the gap between them. IMO the older 24 from late 1990s is a better IQ match for the Tri-Elmars than the newer Karbe-designed 24s or the Zeiss 25.