Hm, underbelly is for sure not a big deal. I can't remember for much images showing a upside down car rendered. So smooth and flat is no problem, I guess.

With a look to this scene photo ufoseries.com/photos/strakerInCar.jpg it seem's that also in reality (movie) there was not very much stuff in the rear part of the car ...

B.t.w. look at that silly telescopic hydraulic door opener at the right side ... I think, also in the pre-70th there must have been better ideas ... :laugh:

But if you need help or an example how to get such a thing working in poser, don't hesitate to ask. I have done such a telescope thing with a morph and some combined ERC's to hold all in line and have the impression of a smooth movement ... it's a bit tricky with masters and slaves and dummy slaves and such stuff ... valueparm work

In that case I would do the door as one actor rotating at their upper fixation. And the telescop is a second actor rotating round it's lower fixation. The telescope is build as total length first and gets a morph for it'sshortes length. And than with a number of time-delayed valueparms both are rotated in a manner to hold the top point of the telescope at the place there it seem's to fixed at the door. So the full thingy is dialed with one master rotating the door down and the telescope inside while also shortening the morph. As said a bit tricky but possible and well working. The number of dummy parms is only depending on the maximum length of linear actions.

Though, with a second look to the image, this telescope doesn't work in reality. It's foot point is located too far outside. If door is coming down the telescope is coming to a horizontal position and for that it have to reside much more inside of the car or the value of power will go up through the ceiling ...