"An IQ smaller than his hat size"

Also said with "boot" instead of "hat". [Used to be a criterion for joining the British National Party, though they do wear big boots!]One I like is:
"half a bubble off plumb" (from a spirit level)

On a similar track there are the expressions of the form:
As much use as:
a chocolate poker/fireguard/teapot
an inflatable dartboard

Couldn't fight his way out of a wet paper bag
Couldn't organise a piss-up in a brewery

Like most similes and metaphors, these are best heard fresh or at least infrequently; new ones can be an absolute delight.
Rod