Probably the Best way, but there may be other more practical solutions, would be to use a piece of plywood, wrapped with visqueen and screwed to the interior framing of the opening with some foam gasket air conditioning tape to create a buffer for weather tightness at the perimeters.
Otherwise, just staple a double folded sheet of plastic visqueen to the framing, but you will probably have to replace it every few years due to UV degradation.
Not to hijack the thread, I think this is relevant: what is the best way of closing off the gable vents when a ridge vent is a retrofit? Would metal duct tape applied to the inside of the gable vent last?
pull up a section of the decking
have our smallest, wire-iest guy wiggle in with a tape measure
scream out the dimensions
on the roof, someone would cut a piece of OSB & drop it in