Here's a design for a basic submarine base that we used on Sunday:
Build costs (including TC):
Fortify file: https://cdn.discordapp.com/attachments/280366398197465088/978262627132264479/sub_base.txt
This design uses a garage door. If you use a double door instead it'll cost 100 less metal, but you'll also need to remove and replace the door every time you bring the sub in and out.
If you build it so that the floor of the triangle foundations is just above the waterline that should be deep enough for the sub. You can see me building the base at 8:33:17 in this video: https://www.youtube.com/watch?v=yHq8NzwZEh4
We tried building this on the edge of an iceberg, but it seemed impossible because their edges are a sheer drop into the water. You need ground to place the foundations on.
Note that the window in front of the TC is supposed to be filled in to make the TC more expensive to reach. You remove the glass with a hammer each time you want to access the TC.
There's barely space in this base to place two sleeping bags but you could expand it outwards from the front door as needed.