Recoring fuel tank cover on 23 sceptre

[JohnB]

I pulled the fuel tank cover, and after visual inspection, decided to recore it. They did a terrible job doing the original core. The edges had air gaps all over one side that let water at it, and there where alot of areas in the center that did not have enough glass on them, and it did not bond well to the skin. The original core was balsa, and here is my question (finally).
I have 3 options for coring material,
1. Balsa
2. Plywood
3. Foam core
Does anyone have any suggestions or observations in doing this with any of the 3 materials above? The balsa would of held in fine if it was done correctly.
Anyways, what to the resident experts suggest?