Hello Ian,
I have thought about the same issue and a solid chunk of timber is nice, but requires the sort of specialised jigs you show or CNC machining.
Another possibility, if you can find one large enough, is to use a large wooden bowl perhaps 700-1000mm in diameter and cut it into quarters. OK- you don't have a great say in the profile, but the constuction is much simpler as you just need to glue on straight sides for a 90degree flare. Then to make the whole thing "like a rock" fill the dished top and bottom with filler/timber whatever- even polyconcrete.
PS: I'm still looking for the bowl
- in the meantime I thought I might settle on a large commercial horn flare (pref. in Australia)....looking looking...
Best
JA