Well, a "dish" might not be needed at the relevant frequencies. Also given the size of the target, not a lot of focusing is needed.

As for the finals, being in a fair vacuum is helpful. You can make *monstrous* tubes, and it's fairly easy to "pump" them to a good vacuum. Also you don't need strong walls as the pressure difference between inside and outside is fairly trivial in terms of loading forces.

Probably need more reinforcement to protect from micrometeorites and idiots running into them.

A multi-megawatt (gigawatt?) tube will be a sight to see.