Personally, this is one of my favorite conversions. I just love this little guy! In my head, as juvenile as it may sound, I just imagine the Ghost in the Shell tachikoma voices going "Teehee, Skulls for the Skull Throne!"
This little guy was made from a Tau Piranha with Defiler body. The back "hatch/cargo" area was made from another item I love using: PVC piping (in this case the end mount).
You can find lots of great stuff at your local ACE Hardware or equivalent. I believe the hatch here set me back some 88 cents.