It's similar to an idea i had back in 2238 but i would never suggest it due to assumed high complexity. In my version, it was a "Base-Boy" or Build-Boy upgrade to Pip-Boy. I pictured it as having its own interface, so you know you're looking at some computerized feature by the looks of it. You place wall segments and floor tiles, choose roofs, set doors, drop in furniture and ground effects, but it's all just in your computer so it's in plan-mode. And the whole time the Build-Boy gives you a running total at the bottom of how many resources and caps it will take to build what you are planning. The number just keeps going up and up as you build more and more into a map. Finally, you press Save, and name it. Your new custom base becomes an in-game file, which you can choose next time you site a base.
But the only way i see such a thing working is some super-awesome dev would need to code a tool that converts in-game data (all those choices you just made) into a real map file when you press Save.
Later, you site the base, choose your file, and you get a typical construction site. You bring the mats there, click terminal (i guess), and BAM! you now have your custom base.
Is that anything like what you mean?