I used an EPS foam blocks that I think I found either at West Marine or at
Defender.

Nowadays, I would look at the TotalBoat system (structural density), easily
found at Amazon:
https://www.amazon.com/Closed-Cell-Foam-Flotation/s?ie=UTF8&page=1&rh=i%3Aap
s%2Ck%3AClosed%20Cell%20Foam%20Flotation

Just be sure that the product is "closed cell" to keep out water.

Mike
s/v Wind Lass ('91)
Nissequogue River, NY