The cube is smaller than it looks. It’s a bit bigger than the GameCube. 3D print a handle for it and it’s as portable as a machine gets!

Edit: I mean, obviously not as portable as the Deck, but that’s a handheld.

Edit 2:
Steam Machine is 16.2 x 15.6 x 15.2 cm.
GameCube is 15 x 15 x 11 cm.