not as uncommon as you may think,alot of home built aussie yachts used sydney
lamp posts as masts.
though care is needed when attaching fittings to the alumillium,and preventing corrosion
generally 5-6 mm wall thickness is good for a 8-10m mast
though would have thought finding a second hand mast
from a 30 footer,including the main sail might be more cost effective,check out boat yards for deals on scraped/wrecked yachts.