[eqvinox]

You get your private prefix by appending 10 random hex nibbles to "fd". For example, fdb9:3667:a8da::/48. (Please make your own random prefix.)

You also missed the point of this sub-thread since the question was about private space. IPv6 fd00::/8 is IPv4 10.0/8. The only difference is an added bit of semantics to make merging networks easier.

[iso1210]

The problem is that ipv6 advocates say "just have your network auto-renumber itself every time you change upstream provider", "run multiple IP addresses on every host" etc

I don't think I've ever seen anyone suggesting using a /48 in fd00:: and NAT66 at the edge, and NAT64 is discouraged rather than maintaining a dual stack solution (and if you need to maintain a working ipv4 solution, why even bother with ipv6)