As Greg says go for the lightest you can get,hotwire or laminate the rocker on a table
If you hotwire,which is the simplest way,make two templates that are a cross section through the center of the board,stick them each side of the eps block
Cut the bottom rocker first,now turn it over draw your plan shape on it and then shape what ever channels concaves etc
Doing it this way makes it real easy as you are working on a big stable block if you mess up unstick the templates and move them down 10mm rehotwire and start again
For bottom the shapes use only sandpaper on a board 100 grit gentle touch clean off the sand paper every couple of strokes
for concaves make a curved sanding board
Once the bottom is done turn it over use the off cut as a rocker table and then hot wire the top
cut the plan shape,remember to deduct the rails
all you have to do now is the deck roll measure and mark then surform down then 80 grit isanding board finish with 100 grit sanding board Once again use the rocker table left over from hotwiring to work on
That way the board is allways stable
You will find that its just like working a pu blank when you get the hang of it
If you have air blow the surface clean regularly while shaping
You shape the rails after the foam skins are on using a sanding block
I did a board a while ago with 5mm 90 kgm3 skins and 90 kg m3 rails and 3 oz glass each side two layers on the deck It weights just 2.5 kg with fins and is so strong its crazy Im going to do another soon this time i will only put a single 3oz on the deck
How does it ride?
I like it a lot, it maybe has not got quite the spring of the balsa or beech boards but it does have more spring than my pu epoxys
The lightness is the key though for me it feels a lot easier to put it in different places on the wave face
Mike