I understand that you wish to randomize the delay caused by the Transport Delay Block. In the Transport Delay block, you can see a parameter 'Time delay'. You could use a random number generating function to specify the random delay you would like to have.
For example, if you keep the value of 'Time delay' as 'rand' it will return a single uniformly distributed random number in the interval (0,1) for the delay .
I hope the above suggestions help!