I can't say I have been building boats for 25+ years, but the tips I suggested at worse will not hurt anything if tried. It is all to the plus side.

The hairspray will dissolve and fuse with the epoxy as it sets. Once the epoxy has fused and cured itself with it and is in the wood, water has no effect on it.

The balsa glue blocks in the corner joints of the bulkheads or anywhere else you see fit can only be a benifit on making sure the epoxy stays where it should without having to build up a large fillet of epoxy in hopes to just to fill a gap. The fact that it is balsa doesn't matter. It could even be a creased slip of paper that gets pushed into the corner to do the job. All you're trying to do is dam up the epoxy and keep it in the joint.