A spacer includes opposite first and second portions facing one another, and at least two spaced reinforcing beams connected to the first and second portions and extending transversally with respect to the portions, the reinforcing beams being disposed both on one side of the first and second portions. The spacer further includes at least one abutting beam connected to the first and second portions, extending transversally with respect to the portions and located on the side opposite to the reinforcing beams. The first and second portions delimit together with the reinforcing and abutting beams a pocket configured to receive at least a contact roller.