Prison staff say only two of the 15 guard towers at the jail could be occupied at any given time due to a lack of resources, reports the BBC.
The illusion of an extra guard was created when they put a prison officer's cap on a football in a third tower.
Escaped convicts Walter Pozo and Cesar Andres, who were serving sentences for armed robbery, are still on the run.
"We've made a dummy out of a football and a prison officer's cap, so that the prisoners see its shadow and think they're being watched," an unnamed prison source told local newspaper Diario Rio Negro.
"We named him Wilson, like in the film Cast Away, and put him in one of the towers," the source added, referring to the volleyball Tom Hanks' character used for company.