I use the normal car one (TomTom) mounted above the clocks and in a cheap case which keeps out the rain. Phil mentioned dazzle from the sun so here is my solution to it. An elastic band round the case and brackets keeps it all in place and it works a treat
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This is the sun shield showing the two "L" brackets and backing plate made from thin aluminium sheet and pop riveted to the plastic shield.
Raw material for plastic shield was this
Other brands of washing products are available and possibly in a size to suit your case
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For anyone interested Fizzy Pies has shown detials of his SatNav mounting above the clocks in his "my bike" post in the 600 section, including drawings of the brackets etc. for a boxeye.