If your using a circular saw, just set the blade depth to just under 1/2 " and then finish cutout with a sharp knife. If using jigsaw or reciprocrating saw, make a hole big enough to map out where the joists and other obstructions, plumbing wiring are. Cut to those points and then finish with by hand.
Replacement wood, might be a metric thickness. Check with your local building supply store (Home Depot etc) or you might have to sand it down or and a filler veneer.
Ken
2006 Winnebago Outlook 29B E-450.
2012 Honda CR-V AWD
Blue Ox Aventa LX tow bar and Brake Buddy Vantage.