The rounded curves on the faces can be easily
sculpted with Milliput, although the tentacles on the octopus above
are far better suited to green putty.
Flat shields can be easily augmented with some weird putty
creations as shown here. If you haven't used putty much, this is
a nice easy starting project.
The 'evil face' idea for shields has been blatantly copied from Fraser Gray, who has had
a number of his excellent figures photographed in White Dwarf over
the years. The models in these examples, however, are my own.
The nails stuck into these faces are made from copper
wire with a small dab of putty on the end.
Make a hole for them while the putty is still soft, and then once it
has hardened, superglue the wire in place.