Stability in bouncy car

I have recently built a Donkey Car and attached a plough to it in order to move objects into a designated rectangle. However, I noticed my vehicle is rather bouncy with no way to adjust the springs. Can anyone advise? Thanks in advance.
Kind regards,

You can either replace the springs with stiffer ones, which you would have to source through some industrial supplier, or you can replace them with rigid struts. I went the second route by 3d printer, but you can easily construct something equivalent with only a drill, a hacksaw, and some scraps of plastic, wood, or metal. Be sure the material you choose is strong enough though! I first used some scraps from the housing of an old miniblinds roller, but they eventually buckled.

(This particular design is for a Traxxas Slash.)