A modder has designed and put together what he reasonably describes as a DIY Steam Controller 2, or Steam Deck Controller. For a further useful twist, electrical engineer TommyB also folded in the appeal of the Nintendo JoyCon-like splittable controller(s) in his recently shared OpenSteamDeckController project.
Some might consider the original Steam Controller to have been ahead of its time. However, in 2025, its control scheme with a prominent pair of trackpads is no longer so alien and undesirable. We have the success of the Steam Deck to thank for that.
