A high-pressure pressure vessel for storing natural gas comprises a plurality of first vessel regions of first diameters, a plurality of couplers, and a fiber layer. A three dimensional volume is filled using at least in part the plurality of first vessel regions. Each coupler of the plurality of couplers couples each pair of first vessel regions of the plurality of first vessel regions. Each coupler of the plurality of couplers comprises a second vessel region of a second diameter and two third vessel regions that transition diameters between the first diameter and the second diameter. The three dimensional volume is filled using at least in part the plurality of couplers. The first vessel regions and the couplers comprise a material with low permeability to natural gas. The fiber layer surrounds the plurality of first vessel regions and the plurality of couplers.