Whatever wood you use it will never match all round and always look a mess. Would you not be better to have a piece of Ebonite? That is the black synthetic material that gunsmiths / stockers use to extend stocks. It shapes easily and fits perfectly if you don't need to remove the extension, much more work if you do ! I have a selection of thicknesses and can provide full instructions for fitting and finishing having done hundreds of them for all sorts of stocks. £5 +P&P for 1" thick.