Step one. Seal the paint. Double the manufacturer's recommended drying time before you pour the Stillwater. Any trapped gasses or moisture will affect the transparency of the Stillwater. I don't use GW paint. I do not know if the Stillwater will peel it or lift it or affect it in some other strange way. Make sure it is sealed well and absolutely dry.
Step two the dam. I do not have the model to look at. I believe base is round. So the shape of the dam will be the major issue.
I am not going to recommend clay see above.
I am not going to recommend tape. Mostly because of the shape of the dam. I use packing tape all the time for dams. But only if they are square.
Cut a strip of clear plastic the depth that you want to pour the Stillwater. Following the contour of the base temporarily attach the plastic. This will form a clean edge for the water. The Stillwater is thin and will leak. So create a backup for the plastic edge with clear silicon window seal. Clear is important so that you will be able to detect leaks. The window seal will be relatively easily to remove when the project is dry.
Step three. The Stillwater shrinks and you may have to make several pours. You can reduce the capillary effects with a thin border of white glue at the water edge. You might want to do the same thing with the dam edge to reduce the amount of sanding you have to do at the edge.
There is one more thing to consider. The Stillwater will generate some heat. Will it be enough to adversely affect the Find Cast. It will depend on the volume used. All of my experiences with fine cast are bad.I do not have this model. My experience with Stillwater is mixed. There are other products I like better.
If you don't need a deep pour you might consider a few coats of gloss Mod Podge.
I spent a lot of time trying to answer a involved general question. You indicated you were going to experiment with the product first. That is a good plan. I hope my answer helps
