Hi Soggy,

The foam I used was a readily available high density blue modelling foam and proved to be very light, quite strong and effective for the task despite its regular abuse. As sugested by others already, use the rear face of the slot as the starting reference since you need the centreboards as far back as possible. If the rear case is already raked back at 5 degrees then you don't need a rear wedge; just a strip of rubber glued to the solid casing. This assumes that your centreboards are ancient and have square edges front and rear. If they have a uniform section over the whole length then you may need to hollow out a "Vee" section of foam at the rear.

If the rake is more than 5 degrees then use the bottom rear corner as your starting reference and make up wedges to suit.

Good luck with your calculations.