I can't locate a supplier for a reamer with sufficient depth...so...any ideas?
Would have thought if you did it might prove to be very expensive, not exactly common sizes.
I suspect you will have to make your and I think a simple blade reamer should do especially if you rough turn as much as your able. The saving grace is that it lignum and not some other less helpful timber.
Using spade bits is ok, except here the depth is a problem, you might need several covering various sections of the tapered hole, could be messy.
If your able to locate some 3mm 'ground stock' or 'gauge plate' from a local engineering supplies
https://maccmodels.co.uk/index.php?cPath=286Cut a flat plate to the dimensions of your hole, mount it on a suitable rod to fit into your chuck on the tailstock. You will need to put some relief angle on the blade edges prior to hardening around 5-7' should do it. This gauge plate can be easily hardened with a good blow torch etc and quenched in water. Just heat it up to a 'cherry red' and dip into the water vertically so as to minimise any distortion. This will produce a very hard blade, ideally some tempering would be nice. But if your not so familiar with this process just leave hard.
Stone up the two edges till razor sharp, bearing in mind its carbon steel and heat will effect the hardness. Low revs and patience it should work out fine.