I try out an alternative material for the funnel. The
tin foil is easy to use, but it creases up too easily,
scattering light in lots of different directions. So
I cut a funnel out of thicker metal. It's a bit of a
nightmare to handle, but will do the job better I think.
I also try out a black pot, between glass sheets, inside
an inner tube. It actually worked quite well, but nowhere
near boiling. Hmm ... this may be a harder challenge
than I'd thought.