So in roughly chronological order, here are the custom stadiums I could find with links to their threads.
DM's Hyperpitch 2.5

Virral's Portable Board

Jeii's Custom Board

Drakeular's Stadium

GrimRod's OrcaCola Stadium

Reaper's Black Pass Pit

Virral's Magnetic Stadium

PianoDan's Tsunami Stadium

Borzag's Pylondome

Virral's Commission for Spieroz

Aspie's Snake Pit

Virral's Dwarven Board

Polemarch's Awesome Trio

PullsyJr's Custom Pitch

Reaper's Sandgroper Pitch

PullsyJr's Custom Pitch 2

Greywolf's Foldable Pitch

StoneColdSpider's Factory of Pain

PullsyJr's Custom Pitch 3

Reaper's Cthulu 7's Board

Ironside's Swamp Board

Virral's 40mm Megapitch
