Depends on the amount of dirt and level of unevenness, really.
Is the sump basin all plastic, another material, or is it earth on any of the sides?
You could use fabric (be sure to use filter fabric or ensure its permiable), but depends where you're thinking since you want to be sure not to plug up any of the inlets (again depends on style of basin) to prevent water from flowing in.
If it is just dirty from built up....dirt, could you shop-vac it clean to level it out and clean it?
You could also put the pump in, level it (either by leveling below it or otherwise shimming it) and then backfilling with some rocks that allow good drainage a couple inches (well below the float). That will help hold the unit in place, also help filter any debris before it gets to the pump, and possibly help with leveling.
https://www.youtube.com/watch?v=5nBqjqCLbj0