I use either brass or plastic spacer pillars.
You can also use just long screws through from the outside of the box, with a nut to lock it in place then more nuts higher up to rest the item on and clamp in solid.
To get the holes accurate, hold the board or module in place inside the box then spin a drill bit through each hole using your fingers, to put a centre mark on the enclosure base or wall.
Some electrical enclosures have screw bosses built in that you may be able to use.
You can use self-adhesive cable tie bases to fix wiring in place, or "harness" them with ties or lacing cord, as long as you do not have high voltage or high power mixed with small signal wires - they should be kept well apart.
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This is a control box for one of my 3D printers; it shows cable harnessing with different bundles for different power and signal wiring, and the threaded spacers supporting the boards:
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The case type that is in would be quite nice for audio gear - a Takachi YM series, 400mm x 250mm; the same style is available in multiple sizes down to 50mm wide.