Thanks Morton, Gerard, Adam, Steve and Bob!
Steve, I did do the math for the pole length, but seeing that this called for a ~2.5m length in front of the OTA I decided that was going to be impractical in the field. At home or at the observatory that may have been a different story. You could attach the thing to the dome and then move that, or a separate mount or something. Anyway the result was a 1.5m-compromise, where I added a curved lip to the cardboard shield I had used for the previous Venus shots from home
2 days earlier, which would end up taking a "bite" out of of the scope's aperture. I reckoned this reduction would mainly impact light grasp (not a big deal for this target) but much less resolution, since the full width of the primary was still clear along the other axis, and daytime seeing isn't that great to begin with. The size of the shield was arrived at by trial and error.