The build is finished! We printed the missing parts, re-printed a few (don't mix high printing speeds with low infills!) and painted and trimmed the tube to the right height.
We had to re-design the tube holder, since we initially went for a combination of
off-the-shelf PVC and metal adapters held together by a rubber fitting which of course
failed miserably, so we followed the KISS methodology and printed a single
piece that did the job. Below we can see what we expected to happen and what actually happened.
This also presents the advantage of portability: the two 3D-printed parts holding the
tube in place do so by friction, so while in the future we may want to glue the pieces
together to make it more robust, for now we can disassemble the robot in three
convenient pieces for transport.