This sphere-like shape has 20 triangular faces instead of 64 quadrilateral faces; ∴, it can be manipulated, stretched, & hollowed.
This download can be saved directly to the prefab folder (default: C:\Program Files\Steam\SteamApps\common\Team Fortress 2\bin\Prefabs). From there, it can be placed in a map like any other primitive shape (block, spike, cylinder, sphere,…). Just choose “prefabs” from the drop-down list that's normally set to “primitives” & select “icosahedron” from the list that normally ∋ “block”, “cylinder”,… as options.
The texture of the icosahedron is NODRAW until it's changed, regardless of what texture is selected when it's created.
Do ¬ stretch the icosahedron too far; it could cause the map to crash on loading if it's stretched too far. If this happens, it can be corrected by deleting the icosahedron & replacing it with a less stretched icosahedron.
The examples in the pictures are in compiled form here: Icodemo37.bsp