Why do you need a CRS? If you don't set one in Qgis then Qgis will just assume your coordinates are on a plane. You won't be able to do things like convert them to lat-long, but then your world isn't on our world!
Is that 30-thousand km or 30-decimal-zero km?
If 30 km, then you could just keep your coordinates in metres and use British Ordnance Survey Grid references, and then your world would be a little square off the south-west coast of England if you tried to project it to a real map. But you don't need to give it a CRS unless you want to work with other data with other CRS systems, which I don't think you do.
Or... is this an entire planet, and you want it to be spherical, and you want to calculate distances between thing along great circles? Then you'd need to define a CRS with a different radius (or elliptical radius and eccentricity).
But otherwise, don't bother.