Through-patches are needed sometimes and rather than trying to fit a patch to perfectly match the outside, I tend to use shavings first and then fit a patch from behind, all supported by a plaster cast.
I feather the edges down to nothing by creating a concave curve from the inside using gouges, scrapers and sandpaper and then press a shaving in with a hard spongy material shaped to the right size and supported by a wooden block from behind. A rugged edge is easier to hide than a perfectly straight edge along the joint of the original.
I locate the shavings with masking tape, making sure the grain lines match up perfectly on the outside. If the shavings are very thin, I press another softgrain shaving onto the wider area to strengthen it, then smooth it off before fitting a patch from behind.
Iris Carr Restorations 