"Cage-like" Formazine particles, how are molecules arranged?
First, note that


formazine structure attached from PubChem is a different compound;
note that there are also, related, formazan $\ce{HN=N-CH=N-NH2}$ and “formaldazine” $\ce{CH2=N-N=CH2}$ (dimethylidenehydrazine);
formazin (polymer) does not necessarily has to have cage-like structure, as this characterization belongs to hexamethylenetetramine in the quoted citation.


Hexamethylenetetramine (urotropine, $\ce{(CH2)6N4}$) can be prepared from formaldehyde and ammonia. In water it also somewhat decomposes to its precursors:



Formazin (also called formalazine) structure might be still unknown, as a related publication from 1976 states:[1]


  … This substance, an insoluble condensation product of uncertain composition, is prepared by mixing solutions of hexamethylenetetramine and hydrazine sulfate. …


Sometimes empirical-like formula $\ce{(C2H4N2)_n}$ is claimed.[2]

Sometimes, claimed structure like following one is presented:[3,4]



It was (with several other structures, less nice or likely), hypothesized (well, rather more complicated, with some ether linkages $\ce{-CH2-O-CH2\bond{-}}$ and $\ce{-NH\bond{-}}$ groups present), based on spectroscopic studies.[5,6]



References:


Rice, E. W. The preparation of formazin standards for nephelometry. Analytica Chimica Acta 1976, 87 (1), 251–253.
Ziegler, A. Issues Related to Use of Turbidity Measurements as a Surrogate for Suspended Sediment. Turbidity and Other Sediment Surrogates Workshop; Reno, NV, USA, 2002.
Kaur, N.; Kishore, D. An Insight into Hexamethylenetetramine: A Versatile Reagent in Organic Synthesis. J. Iran. Chem. Soc. 2013, 10 (6), 1193–1228.
Sadar, M. J. Stabilized Formazin Composition. US Patent 5,777,011, July 1998. 
Mashima, M. The Infrared Absorption Spectra of the Condensation Products of Formaldehyde with Hydrazine. Bull. Chem. Soc. Jpn. 1966, 39 (3), 504–506.
Bondybey, V. E.; Nibler, J. W. Infrared and Raman Spectra of Formaldazine. Spectrochimica Acta Part A: Molecular Spectroscopy 1973, 29 (4), 645–658.