That depends on what tools you have. I've built a deck entirely with simple power tools and hand tools, but any kind of cnc machine like a 3d printer will make your build 10x easier.

I'd start with a pelican case build, because it already covers difficult parts like case, hinges, and locking mechanism. Cover frames for display and keyboard can be cut from abs sheets with simple hand tools, or drill/ angle grinder/ hand file, if you're working with metal.

Breakout connectors can be bought where you get parts for boats or cars.

An internal frame to hold components can be made from plywood or multiplex.