The only problem is that IRL you needed batteries on both sides to close the gulf. No way to do that with this engine I fear.
Do we have any info on how wide the gulf is at its most narrow point? 80-100 km? Its not unrealistic for Estonia and Finland to blockade it if they work together.
Have the AI of Finland and Estonia build them on their respective sides, and when Estonia falls, Finland can extend their coastal artillery to Islands. I believe this would also make Finland a lot more fun to play. And would add more strategic depth to naval warfare.