I decided to try using modelling clay to fashion the onion dome for the tower. A short length of dowelling was added to the cupola top and this was used much like a potter's wheel to fashion the basic shape of the dome. Some years ago I picked up a profile gauge (shown) for a bargain price. This handy little gadget has an array of fine steel bars which slide within the ruled casing, and allows the profile of any object to be recorded. I'm using this to fine-tune the tower shape so the profile matches all round.
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The next step will be to smooth the surface then score shallow lines over the dome to mimic the effect of copper plating. A couple of coats of diluted PVA or varnish will be brushed on, folowed by the paint and more varnish. The clay can chip or crumble if knocked so this will help preserve it.