I believe the interpretation of the GM20.2 (Robot may not intentionally attach to the upper or lower plywood bases of the goals) really depends on the definition of 'attach'. If the contact with the plywood results in the abillity to control the goal, then it's definititely illegal. In your case, if resting the plywood on the robot restricts the goal's free movement, then I believe it's illegal. Sorry if that hinders some of your plans.
~Hubicki~