P clips screwed into a sheet of plywood will attach the wood platform to the rear rack.

then see if the pizza parlor will, sell you one of their delivery bags..
that is unless you are doing a Pizza by bike delivery then they will throw that in with your Job.

another thing is Porteur racks , style is hot these days in some circles , French newspaper delivery bikes 20 pound bundles..
from presses to the street corner kiosks where they got sold

Big front top of the wheel rack..