Paul, I contemplated one of these years ago and had the same thoughts. How large are they?
Deoderant (or similar – look in boots) clear plastic tops are possibly an option, or i seem to remem,ber thinking that 15Watt bulbs were about the right size (minus the metal fitting!)
Plastic forming would be the ideal, but I have never had good results with this, or making a segmented dome (like a banana skin)….or casting resin to the shape and polishing it if weight is not an issue..
Do Deans marine or another supplier make ready made domes of the approx size
Ashley