I will be interested in hearing hear how the Trex holds up. It is a composition of recycled plastic and wood fiber (sawdust?) I have worked with the stuff and am not certain it will stand up to the rigors that a grouser pad demands. I say give it a go. You won't know if you don't try it!    <!-- s:wink: --><img src="{SMILIES_PATH}/icon_wink.gif" alt=":wink:" title="Wink" /><!-- s:wink: --> 
   I always figured I would get some used  payloader tires and cut them up with a sawzall. You might try used  over-the-road truck tires too, but I am not sure they would be thick enough to stand above the grousers on the track. Check with a local tire dealer near you.The local tire dealer here handles a lot of heavy tires and always has a stack of old used stuff and I am sure they would be delighted to give them away.
  Just my thoughts,
             Cal