First, forget about making a lightweight board from soft xps. It's not the right starting point.
Putting glass between the foam is useless. You get stiffness from skins.
6cm is way too thick. A board 20-40 mm is better. Shape it down. I'd just use one 30mm.
The soft xps needs a higher density sandwich on deck. That can be wood or other HD foam you can buy. Glass under, then 3x glass over. 3x glass bottom.
Carbon is waste if money if you are building boards for first time.
In think you need stringers for and around foil attachment. or substantial blocks and fiber reinforcement as bwd says.
TomW, Are you referring to low compressive strength XPS when you say 'soft xps'?
I've read on some boat building forums about sandwich constructions using 400kpa compressive strength XPS foam (Bunning's Knauf Multi-purpose board is 300kpa) and one source I came across claimed they'd run the numbers and for 400kpa 25mm thick panel then with glass laminate thickness greater than 0.72 mm (c. 3 layers of 6 oz) the core will fail before the laminate because the shear strength of the XPS is poor (compared to PVC foams). Thicker panel makes the problem worse because the tension at the surface of the panel increases under flex as panel thickness increases.
On a practical note I've used 30mm xps foam for my 120 x 42cm board and there is more than enough volume to have it always floating (previous modified twin tip took a long time to resurface after a nose dive) so there might not be much to gain from a thicker board if your going with XPS.
I've got a new board planned and will probably use stringers again to support foil connection and footstrap inserts. Although I've never built one without stringers I just know from my previous board that hitting a sandbank at reason speed was enough to crack a 12mm paulownia, glassed twin-tip style board
at the mounts. If your only going to kite in deep water then maybe you don't need to built for the worst case but where I live the shifting ocean floor means that I still discover new submerged obstacles all the time so I build above and beyond rather than again and again.
Woodie Cookie in the Netherlands, an ex-raceboard champ, board designer and now foil designer, has made some hollow raceboards. Can anyone think of a good reason why hollow boards wouldn't work for foils?