weapon_cubemap
weapon_cubemap is a weapon entity available in older Source engine games. It has been stripped out in games running on newer Source engine branches, such as Left 4 Dead 2 and Counter-Strike: Global Offensive for an unknown reason.
Fortunately, some games have this weapon restored through mods:
- Counter-Strike Global Offensive - by TopHattWaffle and Zool Smith
- Left 4 Dead 2 - by MrFunreal

weapon_cubemap
exists in Portal 2, but is missing its model and materials, so these will need to be ported from another game.
weapon_cubemap
, you could use a rifle scope, such as the hunting rifle in 
Entity description

This entity gives the player a set of spheres (models/shadertest/envballs.mdl
), each with different reflective surfaces. It is used for checking cubemaps.
The commands to receive the weapon_cubemap (typed into the console) is either impulse 81
or give weapon_cubemap
.
These commands are considered cheats, meaning that sv_cheats must be set to 1, or developer mode must be enabled, for them to work.
You can switch to this weapon (after having received it) by typing use weapon_cubemap
.

bind "X" "impulse 81"; "use weapon_cubemap"
will bind it to the X key. (You may need to press it twice the first time you use it in a map.)