And keep in mind it's normally a commercial ship. As such, you build it in a commercial yard.
Those are a lot bigger, you can expect (if you start with two fresh built yards) to get the commercial yard to 260k tons (enough for module+armor, bridge, engineering space) by the time your military yard reaches around 40-50k tons, maybe earlier.
As a stop gap, building them as a PDC does good enough, they'll probably have some substantial armor all on their own, and if you focus your industry, are quick to build as well.
And yes, you'll then just produce enough pieces and transport them with your freighters, one by one.
Just keep in mind it's rather hard to relocate them once build.