The Powermac G5 Cube was a unique aesthetic design. I found one on Craigslist for an extremely low price and decided to combine it with my then-unused Mac Mini (Mid-2007 Core2Duo 1.83) and make a more modern, usable machine.
First, all the guts were carefully removed leaving only the chassis. Some metal needed to be cut through the bottom of the G5 cage to access the new ports. Thanks to Kevin R. for help on that one.
The center of the G5 cube was one very large heat sink that had to be cut with a hacksaw to make space but preserve the original sliding locks.
Assembly was relatively easy after that - fitting a second 750GB hard drive over a Firewire connection and bridging a Silverstone slot-loading ITX form factor DVD-RW using a JAE50 to SATA adapter harvested from a laptop drive caddy.