(Click on the pictures for a larger version)
The Super Skrull base form was made from an Eclipso figure with some repositioning, leg-shortening, sanding, and chin-sculpting. The unique thing about this custom is that I made it with interchangeable parts to represent the Super Skrull's ability to mimic the powers of the Fantastic Four. Here he is shown using all four powers (stretching, rock-skin, flaming, and invisibility) at once.
I made a set of "stretch" arms from epoxy clay and Magic Sculp with DCD Plastic Man hands. The flames are pieces from the Super Nova base painted with translucent Tamiya paints. My wife suggested that the orange/red highlights looked better than the plain yellow flames (as shown in the comics).
I also made a set of rocky Thing-type arms using the Thing III animated figure:
Here you can see all of the interchangeable parts, which if they could be combined together, would make one really freaky figure!
The arm connections were made using K'Nex parts imbedded in each corresponding mating part, as suggested by fellow customizer Emerald Sentinel.
The entire lower torso is interchangeable with "invisible legs, taken from the clear Space Ghost figure. Each set of legs has a metal snap which connects it to the upper torso. This idea came from Bruenor - thanks!