Thread: What to add?
I wouldn't call it cheap, but the Black Noise Cosmos would fill that 12hp right up and comes in a DIY kit. It's sort of an endless patching wormhole/analog computer thing like Maths or the Serge world. If you like eclectic and unique concepts like the NLC modules, it is an option.
If that doesn't suit you, I second the matrix mixer, but if I were you, I would only get the AI Synthesis one if it's going to be in the top corners or if you plan to arrange it upside down. Otherwise it'll just be buried behind two rows of cables.