temporary roofing repair
I would think that some rolled roofing installed in horizontal overlapping strips starting from the lower edge of the leak area would be a much better temporary solution than mucking around with the plywood idea. 

Rolled roofing will be much better to seal to the existing shingles at the ends of the strips and along the top edge of the upper most run. It may very well be lower cost that the plywood and is certainly more adaptable to size of area to be covered in a temporary way. 

Rolled roofing is normally installed in such a way that the lower edge is top face nailed with exposed nails. The upper edge is then nailed but the next strip comes along with some overlap and will cover up that upper row of nails.

Use good old google to search with "rolled roofing installation" for links to sights and videos on the subject. 

Note: If your roof has large areas with severely rotted roof decking use great care when navigating around on the roof to avoid crashing through and risk of serious injury.