-Montezuma Skull
-SW Target Battle Droid body
-SW Battle Droid arms and legs
Lt Grey spray paint

Been trying to work this one out for awhile.

Anyway I wanted to take a crack at it.

Mine started with the skull head of Montezuma. Hey, it seemed like a natural. The body, however, is very limited in poses. So I started looking for something else that seemed...boney?

Which lead me to Star Wars Battle Droids. The body is a ball jointed red Target Arena Droid turned around so the back is the front and the double antenna clipped off.

The rest is a tan? Battle droid arms and legs popped out and put in backwards. I had to do a little dremmeling as I couldn't find a tan ball joint droid readily but one of the Clone Wars ones was usable.

The red body has the advantage of a neck that slides inside the body which makes the neck adjustable so it doesn't look so long or so short.

The arms legs and head are lightly sprayed with light grey spray paint.

I sorta view Skeletron as a Red Shadows style BAT.