If cupola is already attached
to roof, remove top and drill a 3/4" hole in top (if none
exists). For example hole would be at the top of this copper roof
top.
Add a cross piece such as this
inside the cupola (approximately 2" x 4" cut to the inside
dimensions). Drill a 3/4" hole half way through center.
This cross piece can be placed above or below louvers, depending on your
design.
#1 arrow shows the extension
rod going through the top of cupola and down inside to the cross piece
mentioned above. Caulk around the extension rod at roof top so
leaks don't occur. Add finishing nails at #2 arrow in picture.