I have not built this model and the scale is smaller then what I generally work on... but I would suggest that the mg's can be made...
I would suggest a small straw... next time you get a latte... ask for those straws... I would usually use those for the barrel and then place it inside another straw.
I think for this scale of model it will be the outside "cooling" barrel and you can place another tube inside of that...
Hope you can find something that works...Mitch
PS That seems like a challenging model for your second build...wish you the best...