Two central themes in this metaphoric short story are the clash between the individual and society and the insular essence of families. The theme related to the title, "Simulacra," is the clash between the need for doing and the ambiguity of doing: "We do things, but ... the most important elements [of doing] are missing."
What this family does, aside from their "respective employments and occupations," is to undertake jobs that are neither original nor meaningful. They like jobs that "exist just because, simulacra which are completely useless."
In this story, the simulacrum (singular, a representation of something already existing) they undertake is the construction of an "instrument of torture." It has a high platform in front of a gibbet (a gallows styled with a projecting arm with rope at the end), with space on the platform for other forms of punishment, such as beheading. Their simulacrum has two sets of stairs so a priest and criminal might ascend separately. Neighbors gather, some expressing anger, some bewilderment, some curiosity, to watch the construction. Police visit but go their way convinced by the argument that if it's not used, it represents nothing illegal.
Clash between the individual and society and the insular essence of families: The reaction of the neighbors, such as Don Cresta, pits the values and sensibilities of the community at large against those of the extensive, like-minded, individualistic family ("There are a lot of us..."). Several reactions are indicated, including "treats and protests" and "waiting for something." While the crowd seems cowed and somehow restricted in the shadow of the gallows from vigorous protest, the family takes the mutterings of threats as encouragement to continue to put value in their meaningless simulacrum.
Clash between the need for doing and the ambiguity of doing: Julio Cortázar brings out the ambiguity of reality through the choices, pastimes and family traits of the narrator's family. When taken at face value, there is nothing very unusual about a family having a hobby to fill the hours not devoted to earning a living with interest and amusement. But when this family finds encouragement in threats and protests and delights in recreating a shocking spectacle (gallows are noted as a shocking spectacle in society), then the ambiguous nature of doing is foregrounded through both forms of doing: building simulacra and "muttering vague threats ... waiting for something."
("Simulacra" is found in Cronopios and Famas  under translator Paul Blackburn's section heading "Unusual Occupations.")