Turned the master for vac-forming the clear bubble. It was made from a small piece of mahogany type wood, which I bonded onto a larger piece of scrap timber to hold in the chuck whilst its was turned. To get the shape I bench fitted a simple form tool from some 5mm steel plate.
Any nicks or coarse grain was filled with polyester filler, then the whole piece sanded smooth with 500 grit paper, but not polished, as too smooth a finish is deferential to the vac-forming process.
Diameter of the hemisphere is 21mm, which with the added thickness of the formed material will make for a 22mm diameter dome.
I have plenty of clear 0.5mm PET plastic which is good for forming as it melts at a low temperature and doesn't usually require any pre-drying. This plastic tends to come free of charge, as it's product packaging which would normally just end up in the dustbin.