I have made custom door panels using Auveco 808 clips (see link). I think I drilled 1/8 inch holes but before drilling any new holes you should see if the clip will fit in the existing holes. Fit the wood panel to the door and drill the holes. Install the clips through the panel then put your foam and upholstery over it. You won't be able to see the clips through the upholstery. I have used 1/8-in Lauan hardwood for the paneling.
https://autofastenersandclips.com/products/auveco-808-trim-clips?variant=19613607303
Not sure if anyone reads books anymore but the Automotive Upholstery Handbook is a great reference.