Mark,

You guys sure are quick to suggest other people waste their wood!
I don't usually screw into the wood because I want to make them as
large as possible.
Waste not, Want not.

Mount the wood with the side that will become the top of the bowl, on a
face plate. Then your initial turning will be to shape the outside and
add a tenon or recess for the chuck. Screw holes on the top of the bowl
do not matter since they will be removed when the bowl is hollowed.

BTW, in spite of the face plate, bring the live center in contact with
the blank. It will add a bit to the overall holding power and will leave
a center mark that should be useful when rechucking the piece.

Harry