You may be able to hide the seam with clay and get away without having to fair it if you decide to go with Gel. I like to use duratec sanding primer which is a two part primer that is easy to sand to a nice finish. You can Awlgrip over it easily.
The seat boxes I did on the 25 seafari have the following:
2 layers of 1.5 oz matt
1 layer 1708
3/8 core cell
1 layer of 1780
They are very strong, solid, and light. When you put down you core you are going to want to putty in between the gaps around the coreing prior to the last lay up.
Unless you vacuum bag you will have to weigh out each side like you eluded to.
There are lots of tricks for the radius. I have used clay, bondo. cabosil/microballoons, pour foam. For larger radius's clay is impractical so you are going to have to get creative.
strick
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