try
{
    Class<?> clazz = Class.forName("tconstruct.util.player.TPlayerStats");
    Method m = clazz.getDeclaredMethod("get", EntityPlayer.class);
    Object result = m.invoke(null, player);
    Field f = clazz.getDeclaredField("knapsack");
    IInventory knapsack = (IInventory)f.get(result);
} catch(ReflectiveOperationException e) {
    e.printStackTrace();
}