Installation:

-Unzip unit_skyboxHD.pak into your Quake2 folder under ../baseq2/
-Load map "skybox.bsp"
-Open game engine console and execute command "sky 2k/unit1_" for Q2Pro or Q2Re and "sky 1k/unti1_" for R1Q2 or YamagiQ2 clients.
-Enjoy!

Tested for R1Q2, YamagiQ2 and Q2Pro clients.
Also working for Q2RTX, but it looks bad with RTX ON as any default environment maps.
Should work for KMQuake2. Just try to execute 2k or 1k versions of the skybox.