Why anyone would choose Xps as core vs Eps is not understood.
Any breach of water proofing, even tiny dings, and open cell will drink like its 1999!
Closed cell extruded can be bought for next to nothing, or if stronger core is wanted airex, or other can be used.
I, we have built 7 boards or so, would never even consider rails, but then again we know where our STRUT should
be mounted on board, and strapless most of the time, if foot hooks or straps wanted added AFTER wing delta "V" set for placement.
Moment of inertia if understood and correct carbon weight choice makes this just not needed.
I like just a little volume so use 4 cm thick also aides well rounded rails for safety, but 2 cm board, 2 Ks, holding up well even jumped.
1 k Quadri (amazing stuff if understood how to get epoxy correct) as Box or plate layup extending to front foot placement about half board width
has proved effective. Of course exact same lay up top and bottom, vac sack sure!
300g standard carbon used on the rest, although rails can be tricky if one is not patient.
Cut offs from S U Paddles, thin carbon tubes, cut perfect to fit in between top and bottom for thru M6 prevent compression.
All our boards get well used, no failures.