So I've been fiddling about with the sculpting for a couple of days and now have this...
It doesn't look a huge amount like Harrison Ford, but it does look like a person, which is a big plus. It's enough, at least, to encourage me to move ahead with The Plan.
A big tip, should you want to follow my carbonite-building example, is, when you're losing hope, paint the whole thing in one colour. It really helps to show what the finished thing will look like, as opposed to the big mess of white and beige. The paint also held together the nasty, crumbly Poundland Filler.