I’ve finished the clock now, I mounted it to a wooden base with copper loops, secured from underneath with hot glue. I opted in the end for a DS1307 powered clock, hence the little button battery held cunningly to the bottom left. I couldn’t decide the best way to write the firmware, as reading the ds1307 takes a little too long, so it disturbs the persistence of vision effect. It therefore updates the time every minute or so, which looks a lot better.
Code is here: https://github.com/JamesGlanville/VFD
The code is a bit rubbish, as the wires on the VFD are attached semi-randomly to pins on the MAX6921, so there’s a nasty bit of magic code to work out which segments are which.
The digits are really clear in real life, it is just hard to photograph.