Soeren, I think he's just looking for direction. He's probably a soil mechanics or agricultural or geo-related student, so it's understandable to require technical engineering advice.
UMaine, the issue with suspension systems is that they are dependent on two things: the frequency of the vibration, and the mass that needs to be suspended. If you know both, and neither will ever change, you can build a 'perfect' suspension for it.
But lets say one day you want a full box (20 pounds) and the next day you just want half a box (10 pounds). The suspension can't be optimized for both. You could say determine the 'average' amount of dirt for a box, say 15 pounds, and design for that as 'close enough'. In this case, the suspension will be a little too rigid for 10 pounds and too bouncy for 20 pounds - but it still might be 'good enough' depending on how picky you are.
For SoR single-part suspension (which you saw on my ERP robot wheels), the robot weight was well known and not changing, and the frequency was very low.
If you want something 'quick and cheap', I'd recommend it to you. It's only one part from a single sheet of plastic or metal, so very cheap. However you'll need stress analysis software (such as ANSYS) to design it properly, and good machining skills (I used a CNC). I'm a mechanical engineer so I know how to do the calculations . . . if you can find one to join your team, you can have him do it for you.
If you don't have either capability, or an engineer, I suggest making about ~6 different prototypes of similar design but different beam thicknesses using your intuition. Try each one to see how well it works out. You want it to flex and bounce. If it's too flexible, it'll sag under the weight. Not flexible enough (ie rigid), then there will be no suspension.
Another idea would be to design a system that suspends the box using a system of springs. Buy about 10 different types of springs of different k (strength of the spring) values, and try each to see which works best.
hope that helps . . .