We present different configurations of compact CO2 reference gas cells based on hollow-core photonic crystal fibers (HC-PCF) for laser frequency stabilization at 2 um. Gaseous helium permeation through the silica fiber walls was used to fabricate low-pressure CO2 HC-PCF cells, which were hermetically sealed by different means, including fusion splices to silica solid-core fibers and end-caps. The optical properties of HC-PCF cells built using a commercial multimode hollow-core photonic bandgap fiber and recently developed Kagomé HC-PCFs providing quasi-single-mode propagation are compared and discussed. Results of laser frequency stabilization at 2051 nm using these different HC-PCF cells are presented.