Make the holes big enough to get the tube of an expanding foam gun in and squirt a little foam it. When its expanded and hard cut it off flush with the door and sand down. Click on the link below to buy expanding foam, its delivered in a day.
when hole is made push nozzle to back of door and squeeze gently. The foam will stick to the back of the door and as you pull the nozzle out it will fill the gap between back ad hole for a diameter of about 50mm. Obviously if you leave the nozzle in and keep sueezing it will fill the door so pull it out as you squeeze and you should be fine.