do you have a link to the driver/motor kit ?

is your MDF bare, or have you painted it yet ?I
f it's bare, and already built, you can stiffen it considerable by drenching it in ronseal wood rot repairer(less than a £tenner) . It's basically an epoxy type resin that soaks into the wood , or in your case MDF, causing it to be more like a plastic composite, which is much more rigid after it has dried.

Like I said, let us know (with pics- would help) how far you have got, and we / I can have a look and make other suggestions like this , that might help improve things, very easily and cheaply :)

Rich