Obtaining an IPv6 prefix via Prefix Delegation in Xenial

I want to configure an Ubuntu (Xenial) box as an IPv4 and IPv6 router. (I have IPv4 working already.)

I intend to use dnsmasq to distribute globally routable IPv6 addresses via RA. However, to do that, I first need to get the Ubuntu box acting as the router to fetch a prefix via the WAN interface and to assign it to the LAN interface.

What's the best way to do this in Xenial?

I can find old instructions that involve using the WIDE DHCPv6 client, but it appears that the wide-dhcpv6-client package in Xenial is from 2008 and upstream over on SourceForge doesn't have anything more recent, either. It seems scary to use network-facing software that's written in C and has been abandoned by upstream for years.

As of Xenial, what's the recommended way of fetching a Prefix Delegation prefix via the WAN interface and assigning it to the LAN interface?

(In case it matters, the ISP says that it hands out prefixes of /56 from which the customer's router is supposed to prepare potentially multiple /64 subnets.)